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Interfacial assembly of anisotropic core-shell and hollow microgels

Microgels, cross-linked polymers with submicrometer size, are ideal soft model systems. While spherical microgels have been studied extensively, anisotropic microgels have been hardly investigated. In this study, we compare the interfacial deformation and assembly of anisotropic core-shell and hollow microgels. The core-shell microgel consists of an elliptical core of hematite covered with a thin silica layer and a thin shell made of PNiPAM. The hollow microgels were obtained after a two step etching procedure of the inorganic core. The behavior of these microgels at the oil-water interface was investigated in a Langmuir Blodgett trough combined with ex-situ AFM. First, the influence of the architecture of anisotropic microgels on their spreading at the interface was investigated experimentally and by dissipative particle dynamic simulations. Hereby, the importance of the local shell thickness on the lateral and longitudinal interfacial deformation was highlighted as well as the differences between the core-shell and hollow architectures. The shape of the compression isotherms as well as the dimensions, ordering and orientation of the microgels at the different compressions were analysed. Due to their anisotropic shape and stiffness, both anisotropic microgels were found to exhibit significant capillary interactions with a preferential side-to-side assembly leading to stable microgel clusters at low interfacial coverage. Such capillary interactions were found to decrease in the case of the more deformable hollow anisotropic microgels. Consequently, anisotropic hollow microgels were found to distribute more evenly at high surface pressure compared to stiffer core-shell microgels. Our findings emphasize the complex interplay between the colloid design, anisotropy and softness on the interfacial assembly and the opportunities it thus offers to create more complex ordered interfaces.

Microgels based on poly(N -isopropylacrylamide) (PNiPAM) are highly interfacial active, lower the interfacial tension and are strongly deformed at oil-water interfaces. The interfacial phase behavior of such spherical microgels has been studied intensively showing the effect of size and hard cores to the spreading of spherical microgels at the interface and their response to an increase in surface pressure within the Langmuir-Pockels trough. [14][15][16][17][18][19][20][21][22][23][24][25][26] Only few studies investigate the interfacial behavior of particles with an anisotropic shape.
Due to the specific shape, different ordering phenoma become relevant at the interface revealing tip-to-tip (t − t), 27 side-to-side (s − s) [27][28][29] or triangular 28 ordering for ellipsoidal particles. This ordering is a result of capillary interactions caused by a curved contact line to fulfill Young's equation. 30 Hence, the ordering of anisotropically shaped particles is influenced by multiple factors such as the particle shape and dimensions and the interfacial properties. 28,31 The effect of softness onto the interfacial behavior of anisotropically shaped microgels was first introduced by Honda et al. 32 and further developed in our previous study. 33 Hereby, the interfacial behavior of anisotropic core-shell microgels and more especially the influence of the microgel shell on their deformation, ordering and orientation at the interface were discussed. 33 This led to the conclusion that both the anisotropy of the hard core and the softness of the shell are key parameters defining the nearest neighbor ordering, the microgel orientation in respect to the barriers and the prominence of capillary interactions.
Many objects found in nature are soft and, hence, require a completely soft object as model.
This reduces the applicability of core-shell microgels with a rigid core as optimal model system. Hence, a soft but still anisotropically shaped microgel is desirable. This can be achieved by removing the hematite-silica core of anisotropically shaped core-shell microgels leading to hollow microgels. 34 Similar to spherical microgels, 35 the hollow design is expected to have a major influence on the spreading and assembly of anisotropic microgel at the interface. In order to investigate this effect, appropriate anisotropic hollow microgels that remain anisotropic at the interface must therefore be designed.
In this work, we study the influence of the rigid core on the behavior of anisotropic microgels at the fluid decane-water interface by comparing the behavior of a core-shell microgel with a thin shell to the resulting hollow microgel after etching the core. The shape and size of the microgels at the interface, and their collective and individual response to compression are investigated by combining Langmuir-Blodgett deposition and atomic force microscopy (AFM). Dissipative particle dynamics (DPD) simulations were performed to support the experiments and rationalize the interfacial deformation of the individual anisotropic microgels and the importance of the core-shell and hollow microgel design. We further analyzed and discussed the collective assembly and ordering of both systems at the interfaces from low to high compression.

Experimental Section
Materials All materials were used as purchased. The cross-linker BIS (N,N' -methylenebisacrylamide) was purchased from Sigma-Aldrich and the main monomer NIPAm (Nisopropylacrylamide) from Acro organics. The initiator KPS (potassium peroxydisulfate) was bought from Merck. The co-monomer BAC (N,N '-bis(acryloyl)cystamine) was purchased from Alfa Aesar. For the Langmuir-Blodgett trough experiments decane (Merck) , ultrapure water (Astacus 2 , membraPure GmbH, Germany) and propan-2-ol (Merck) were used. The decane was cleaned by filtering it three times over a column of basic Al 2 O 3 (Merck). The third filtering was done just before the measurement.
Synthesis The synthesis of the core-shell microgel (CS − 165) is described in detail in a former study. 33  The solvent was exchanged to obtain a neutral pH after the silica was etched away and the resulting microgels were lyophilizates.

Characterization
A detailed characterization of the microgels is described in former studies. 33,34 The most important microgel characteristics are summarized in Table 1. Langmuir-Blodgett technique. The isotherms and depositions were conducted simultaneously at a decane-water interface in a customized liquid-liquid Langmuir-Pockels trough (KSV NIMA, Biolin Scientific Oy, Finland) equipped with two movable barriers. The exact procedure is described in Ref. 33 The trough was cleaned with ethanol and milli-q water before a clean water-decane interface was established. To this interface, the microgel dispersion to which isopropanol was mixed as spreading agent were added with a syringe. The substrates were cleaned before they were moved through the water-decane interface. The microstructures of the microgels on the substrates were further investigated with atomic force microscopy (AFM).
For the hollow microgel HS −165, it was not possible to obtain the full compression isotherm within one single measurement with the used Langmuir-Blodgett trough. Hence, a compression isotherm had to be measured with an initial surface pressure (Π) larger than zero. The values for the compression isotherm obtained at the highest HS − 165 concentration were rescaled by 5 mN/m each to overlap all three compression isotherms conducted on this system. The original data can be found in the supporting information ( Figure S4).
Atomic force microscopy. Dry-state AFM was performed to visualize the monolayers deposited on the silica substrates. The experiments were carried out in tapping mode using a Image Analysis The program NanoScope Analysis 1.9 was used for the acquisition and leveling of the AFM micrographs. The processed AFM images were analysed with an updated version of the MATLAB routine of Bochenek et al., 20,33 details are provided in the supporting information.

Computer simulations
Method. To assess the difference between the behavior of anisotropic core-shell (CS) and hollow-shell (HS) microgels at the decane-water interface DPD simulations were used. 36 The method was successfully applied and described in detail for the study of anisotropic hybrid core-shell microgels with a different shell thickness. 33 The modeling of the microgels, which have a size of hundreds of nanometers, requires a proper coarse-graining procedure 37 leading to the effective reduction of the gel size. Dealing with the hollow anisotropic microgels, we found it crucial to readjust a coarse-grained relation between the size, softness of the microgel, and surface/volume microgel-solvent interactions. The higher the coarse-graining degree, the higher the sensitivity of the system to the disbalance between surface and volume terms, which is especially noticeable for oblong or hollow objects at the same coarse-grained level. Thus the size-limit effect (see Supporting information, Size-limit effect) must be considered. In the case of regular or hollow spherical microgels the effect is less visible due to the spherical symmetry of the system. The swelling/deswelling process is accompanied by the self-similar change in the size of the microgel and its cavity.
Both, in collapsed and swollen states, the hollow microgel and its cavity are spherically shaped. 35,38,39 This result is reproduced in a fairly wide range of interaction parameters as well as the flexibility of the subchains. 40,41 However, following such a strategy one could notice, that simulation of the hollow ellipsoidal microgels failed and could not reproduce the experimental observations. 34 In particular, different trends of the shape/size evolution of the shell and the void upon collapse above their volume phase transition temperature (VPTT) are observed or extra flattening of the microgel at the interfaces is detected.
In this study, we extended the method used earlier and applied several modifications to improve the modeling of the CS and HS anisotropic microgels in bulk and at the decanewater interface. The first modification is aimed to choose the reasonable coarse-grained level and reproduce interfacial tension value for the pure decane/water interface without the microgel. Decane molecule is treated as a single bead in the system similarly to Ref. 42 The DPD interaction parameters a ij were estimated through the Flory-Huggins interaction parameter matching the infinite dilution activity coefficient of liquids, where i and j ∈(Wwater bead, D -decane bead) see Supporting information, Decane/water interface. Such a choice of parameters at 20 • C provides the value of the interfacial tension of water/decane liquids equal to 51.9 mN/m which is in good agreement with the experimental value 52.33 The second modification is related to the model of the CS −165 and HS −165 microgels. The anisotropic CS microgel is composed of a polymeric shell and a solid core. The beads forming the core and the shell are denoted by C and S, respectively. The dimensions of the hematitesilica core in the experiment were 330±12nm×75±8 nm. The ellipsoidal nanoparticle having similar aspect ratio between the long and short semi-axes 4.4×1 have been created. Since the real aspect ratio of the CS-165 is unknown, we designed three different microgels with the same shell mass and cross-linking density but different initial aspect ratios of the shells L/d = 2.8; 2.5 and 2.2 to capture the peculiarities of the shell dimensions ( Figure S5, Table   S3). The microgel with a shell aspect ratio of 2.8 is featured by a thicker shell at the top of the core compared to its side. The microgel with an aspect ratio of 2.2 has approximately the same thickness of the shell around the solid particle. Hollow microgels were obtained after removal of the solid core from the corresponding core-shell microgels (see Supporting information, Microgel description).
In our previous work, 33 we assumed that independently of the shell thickness CS-samples The third modification provides the way to reproduce the swelling degree of the microgel at chosen coarse-grained level by establishing the relation between the number of the beads per subchain and the subchain flexibility. Similarly to the work by Nikolov et al. 44 in addition to the harmonic potential, a bending angle potential as well as a segmented-repulsive potential (SRP) 45  Detailed information about simulation parameters, modifications and its impact are fully described in Supporting information.
Simulations were performed using a DPD code supplied by Lammps. 46 Simulations of microgel in a solution were performed in the N V T ensemble. Cubic boxes of a constant volume V = L x × L y × L z = 100 × 100 × 100 r 3 c with imposed three-dimensional periodic boundary conditions, containing single microgel immersed into the solvent were simulated for an equilibration phase of 10 6 steps followed by a production phase comprising of 10 6 steps.
Equilibrium was deemed to have been achieved when the cumulative average temperature, pressure, gyration radii reached a plateau. The simulations of microgel at water/decane interface were performed in the N V T ensemble in rectangular box of a constant volume c with periodic boundary conditions in all directions. The box were fulfilled by water and decane beads. Water and decane phase-separate. Water is at the bottom of the simulation box, decane at the top. Note that the z direction of the simulation box is perpendicular to the liquid/liquid interface. The interfacial tension between the two liquids is determined using the pressure tensors as shown in Eq. 1. The microgel was placed close to the water/decane interface. The equilibration run comprised 10 6 steps followed by a production phase comprising of 10 6 steps. The interfacial tension, the gyration tensor and the surface area of the microgel, were averaged over the last 10 6 steps of a total 2 · 10 6 steps.

Computational details
Interfacial tension (IFT). The DPD IFT (γ DP D ) was evaluated by dividing the simulation box into a number of slabs parallel to the interface and obtaining the ensemble average of pressure tensors which can be presented by the following equation: 47 where rescaled z = z/r c and the angle brackets indicate the ensemble average of local dimensionless tensors components (p xx , p yy and p zz ) of x, y and z directions. In application of Eq. 1, the interface between two immiscible fluids is considered as a plane perpendicular to the z-direction. The evaluated γ DP D which is in DPD units and can be converted into conventional or physical IFT which is measured experimentally, by using the conversion fac- Gyration tensor. The gyration tensor was calculated using the followings equation: where is the position vector of each bead, which is considered with respect to the center of mass of the microgel N i=1 s i = 0 , and the overbars denote an average over all the beads in the microgel, N . To get a complete characterization of the elliptic microgels, we computed the eigenvalues of the gyration tensor λ x = X 2 , λ y = Y 2 , λ z = Z 2 as well as eigenvectors T 1 ,T 2 ,T 3 . The gyration tensor is symmetric thus, the Cartesian coordinate system can be found in which it is diagonal. Transformation to the principal axis system diagonalizes S, and we chose the principal axis system in which where we assume that the eigenvalues of S are sorted in ascending order, i.e., λ x ≤ λ y ≤ λ z and x, y and z are the new coordinate axes. These eigenvalues are called the principal moments of the gyration tensor. The first invariant of S gives the squared radius of gyration, a measure of the average size of the particular conformation of the microgel. We also defined the shape anisotropy of the microgel as Form factor. The form factor P(q) of microgel in bulk was calculated using the followings equation: where the angular brackets indicate an average over different equilibrium configurations of the same microgel and over different orientations of the wavevector q, r ij is the radius vector between the i th and j th beads. The fitting of the form factors were computed with the program sasview v.5.0.4 with a core-shell ellipsoid model.

Results and Discussion
Compression isotherms and interfacial deposition. In this study, a hybrid core-shell microgel (CS − 165) and its corresponding hollow counterpart (HS − 165), obtained after its core dissolution, were employed to investigated the influence of microgel architecture on the interfacial deformation and assembly of anisotropic microgels. Core-shell microgels with larger and softer shells introduced in previous studies, 33,34 did not exhibit any anisotropic shell at the interface. Conversely, the microgel CS − 165 with the thinnest and stiffest shell exhibited an anisotropic shape at the interface 33 and was therefore selected for this study to investigate the differences between the behavior of anisotropic core-shell and hollow microgels at a decane-water interface.
The microgels were investigated at the water/decane interface using a Langmuir-Blodgett setup. Subsequently, the dried deposited monolayer was analyzed via atomic force microscopy (AFM) to obtain microstructural information at the different compression states.
The compression isotherms of hollow (HS − 165, brown) and core-shell (CS − 165, orange) microgels in dependence of the mass of the shell per area of the trough are shown in Figure 1.
The AFM phase micrographs in Fig. 1 evidence the anisotropic shape of hollow (left) and core-shell (right) microgels, respectively, in the concentrated (upper row) and diluted (lower row) regions. The AFM images have a size of 3 µm × 3 µm and demonstrate that even in the concentrated state, the microgels appear elongated independently of the presence or absence of the anisotropic core. Hence, the two microgel systems are suitable to identify the effects of the internal architecture on the interfacial behavior of anisotropic microgels. Furthermore, the hard core of core-shell microgels makes deformations such as bending impossible and the elliptical shape is preserved also at high surface pressures. The hollow microgels without the core obtain the freedom to slightly deform which can be observed at high surface coverage.
This means they still have an anisotropic shape but can bend to better fit into observed microgel clusters. The microgels form clusters which do not show a direct contact in the dilute regime. This could be explained by either a polymer layer which is too thin to be visualized with dry-state AFM or by a deswelling of the outer polymer layer due to the drying at the silicate substrate.
Π is reported as function of mass of the shell m shell normalized by the trough area, Considering that both microgels have the same microgel shell, the compression isotherms are therefore compared in respect to the number of microgels adsorbed at the interface. This representation of the data was selected, as the clustering observed for both microgels did not allow for an accurate normalization by the number of microgels observed For the hollow and core-shell microgels, no flipping was expected due to the spreading of the microgel shells. The two-step increases of the surface pressure in the compression isotherms observed for both microgels are more similar to the two-step increase observed for spherical microgels as compared to those reported for hard ellipsoids.
Region I in Fig. 1 is characterized by a constant, zero Π indicating that the concentration The hollow microgel has a more extended region III and only a small increase in surface pressure in region IV. In comparison, the core-shell microgels has a much narrower region III and presents a steeper increase in surface pressure in region IV. For spherical microgels, an enlargement of region III is related to the softness of the microgels that can be either achieved for core-shell microgels with a hard silica core by decreasing the microgel shell crosslinking or by the removing the core. 25,35 Similarly for the anisotropic microgels investigated in this study, HS − 165 appears softer than CS − 165 leading to an enhanced spreading at the interface and an enlargement of region III.
Geisel et al. 35 investigated the compression modulus (C −1 = −A(dΠ/dA)) for spherical hard core-shell and the corresponding hollow microgels. Two maxima were observed for the compression modulus revealing a two step-increase within the compression isotherm.
For spherical microgels, the existence of the hard core did not influence the height of the first maxima, but rather the second maximum. 35 The same accounts for the anisotropically shaped microgels in this study (see SI for further information). The dense anisotropic microgel monolayers show a comparable elasticity as the one reported for spherical microgels. This is inferred by comparing the data of the anisotropic microgels ( Figure S1) with the data obtained for spherical core-shell microgels from Geisel et al. 35 Spherical microgels with a cross-linking density of 5 mol% have the first maximum at Π = 10 mN/m for C −1 = 46-47 mN/m. 35 Although AFM phase images and dynamic light scattering data of the anisotropic microgels 33 pointed to a stiff microgel network, the compression modulus indicates a similar elasticity at the interface as was found for spherical microgels with similar cross-linker content. 35 The onset of the surface pressure increase takes place at lower compression for the hollow as compared to the core-shell microgels. Geisel et al. 35 observed a similar shift when comparing the compression isotherms of spherical core-shell and hollow microgels. This can be explained by the different size of hollow and core-shell microgels at the interface. Height profiles and the determination of the long and short semi-axes are shown in Figure 2 The mean area of one microgel increases from 0.0607±0.0055 µm 2 for CS − 165 to 0.127±0.022 µm 2 for HS − 165. By removing the core, the microgel shell can spread freely at the interface and the area covered by one microgel roughly doubles in size. This increase in area comes from an increase in the microgel length and diameter while the aspect ratio of the microgels stays nearly constant (see Table 2). The length of the microgels increases by 283 nm from CS − 165 to HS − 165 while the additional spreading at the sides is 160 nm from CS − 165 to HS − 165.
.  hollow (HS) ellipsoidal microgels were adsorbed at the water/oil interface from the water phase. It was assumed in earlier studies that CS − 165 microgels have a constant shell thickness, which means that the thickness of the shell at the top and the sides of the core are the same. 33,34 However, this may not be the case. In order to get a deeper understanding on the correlation between the thickness anisotropy of the shell and spreading of the microgel at the interface, three types of core-shell microgels with the same solid core, shell mass and cross-linking density CS 2.8 , CS 2.5 , CS 2.2 as well as corresponding hollow microgels,  Equilibrium structures of the CS and HS anisotropic microgels at the decane/water interface are shown on Figure 3C,D. Decane is a poor solvent and penetration of the oil molecules into the microgels is negligible in comparison with water molecules. The major part of the microgels is located in the water phase, and the narrow polymer/oil interface is slightly bent. After adsorption microgels become significantly stretched due to the minimization of the water/decan surface energy. However, the presence of the solid core and thin, highly cross-linked shell restricts the spreading and preserves anisotropy. Comparing the relative size changes of the HS and CS microgel summarized in Figure 3E, we found out that the effect of shell anisotropy is also accompanied by increasing the surface area. The highest surface area occupied by the HS 2.8 in Figure 3D and gradual decrease of the spreading in a sequence is the following Figure  The heights of the CS and HS anisotropic microgels for the different design are shown in Figure 4C. The microgels show a decrease within the height after etching away of the core. In Figure 4A,B the lateral cross-sections of CS 2.2 and HS 2.2 are presented. The cavity is distinguishable, but highly stretched parallel to the interface. The results are in a good agreement with the experimental observations keeping in mind that the experimental height profiles were captured in the dried state. Indeed, due to the solid silica coated hematite core, the relative difference of height between the two microgel types is much more significant in the dried state compared to the swollen one.  Table 2. The simulation results illustrate the intricate interplay between the microgel design and its spreading at the interface. Due to the presence of the core to which the polymeric network is grafted, the spreading of the CS microgels is strongly constrained. As a consequence, an increase of the shell thickness results in a larger interfacial deformation. This can be observed for CS 2.8 with a much thicker shell at the tip that becomes slightly more anisotropic at the interface. The dimensions of the microgel play however an important role too. Indeed, spherical microgels with similar crosslinking degree flatten more than larger ones which can be explained by higher relative contribution of the interfacial interactions in comparison with the volume contribution similar to that shown in Eq. (1). Consequently, when the thickness at the side of the anisotropic microgels is comparable to the one at the tip as for CS 2.2 , the microgels deform more laterally resulting in a lower aspect ratio at the interface.
For HS microgels, the spreading is less constrained, which can be evidenced by the larger  Figure S13. We distinguish reliable differences in the deformation of the contact line between the CS and HS samples. For microgels with a cavity the fluid interface is quite flat. The trench depth is slightly more than 3σ ( Figure S14B). The difference between the surface area of the interface with (interface with distortion) and without microgel (undisturbed interface) is shown in Figure S14A. The distortion of the fluid interface is rather small. The microgels with solid core create a downward distortion. The polymeric shell tends to spread over the surface, accompanied by downward force that pushes the solid core towards the decane. We observe significant increase both area of deformation and trench depth compared to their counterparts with a cavity, which resulted in the expectation of stronger capillary interactions for the CS anisotropic microgels.
Microstructural analysis of the interfacial assembly. CS − 165, the presence of uncovered areas and thus of a highly inhomogenous assembly of microgels at the interface even at the highest surface pressure (Fig. 5 K). In contrast, the nearly homogeneous distribution (without any uncovered areas) of the hollow microgels at high surface pressures (Fig. 5 D and E) indicates weaker capillary interactions and larger compressibility. This could be an additional effect of the possibility of the soft hollow microgels to bend and therefore slightly change their shape to fit into the microgel structures.
A closer examination of the monolayer topology is shown in Figure 6 and illustrates height profiles of s − s and t − t clusters for HS − 165 (Fig. 6A, B) and CS − 165 (Fig. 6C, D) within the high compressed regions IV (Fig. 6A, C) and V (Fig. 6B, D). Even at high surface pressure, the length of the hollow microgels is larger as compared to the core-shell microgels, similarly to what was found at low surface pressure (see Fig. 2).
The same observation holds for the short axis (d) of the microgels which are shown as   Hence, the presence of the hard, in-compressible core has a major influence on the overall orientation of the core-shell microgels at higher compressions as discussed in more detail in our former study. 33 The larger stiffness of CS − 165 microgels was here responsible for the preferential direction observed for the anisotropic core-shell microgels in region IV and V of the compression isotherm. As both systems present a similar aspect ratio at the interface, we conclude that the stiffness of the anisotropic microgels plays a key role in their orientation under the directional compression set by the trough barriers.
In addition, the nearest neighbor distance N DD was computed within the five regions as summarized in Fig. S3. This analysis first confirms that the average N DD decreased upon compression for the two systems. This effect is attributed to the deformability of the microgels, but also to the promoted s − s assembly at larger compression. Indeed the N DD distributions presented in Fig. S3 show a broader distribution at low compression. This is particularly the case for HS − 165 for which the observed clusters do not present a clear preferential configuration at low compression. Increasing the surface pressure then results in a more compact preferential s − s assembly characterized by the narrowing of the distribution and its decrease to shorter distances. The less deformable CS − 165 present stronger capillary interactions characterized by a preferential s − s assembly already at lower surface compression.
To obtain more detailed information about the short range ordering of the microgels, the s − s and t − t ordering is determined similar to ref. 33 We refer to this study for more details on this analysis. In a nutshell, we considered neighboring microgels presenting the same orientation ± 15°. An s − s configuration was identified if the nearest neighbor center was   s − s ordering are observed in region IV and V only which is the region where no prominent clustering was observed. To conclude, the hard core has an influence on the nearest neighbor ordering but the general type of order is determined by the elliptical shape of the microgels in both cases.

Conclusions
In this work, we investigated the influence of a hard elliptical core on the behavior of anisotropic microgels at a decane-water interface. For this, we prepared first a core-shell microgel with a thin and stiff polymer shell synthesized around an anisotropically shaped inorganic core and then prepared a hollow, anisotropic microgel via etching the core. Comparing the interfacial behavior of core-shell CS and hollow shell HS anisotropic microgels thus having the same shell properties, the examination of the compression isotherms confirms the promoted deformability of the HS system. Both systems maintain an anisotropic shape at the interface with an aspect ratio in the order of 2.
Supported by computer simulations, we highlight how the combination of shape anisotropy and microgel architecture influences the microgel deformation at the interface. We first confirmed that hollow microgels spread more at the interface than the stiffer core-shell microgels.
A closer investigation of the interfacial deformation revealed that the local shell thickness plays a crucial role and CS microgels with a thicker shell at the tip were found to become more anisotropic at interface than in bulk. For HS microgels, similar to capsules, both the local thickness and dimensions of the cavity are key parameters. For our design it turns out that hollow microgels are then conversely to core-shell microgels becoming more elongated when the lateral shell thickness is in the order of the longitudinal one. From the trend, we can assume that this effect will be amplified when the lateral thickness exceeds the longitudinal one. We believe that these conclusions will contribute to a better understanding of the interfacial deformation of soft anisotropic systems with more complex architectures and may be relevant for instance for biological systems such as cells or virus capsids. In addition, our results are expected to motivate the rational design of soft anisotropic objects with a preprogrammed interfacial deformation set by the dimensions of the core/cavity and the softness and local thickness of the polymeric shell.
Interestingly, hollow and core-shell anisotropic microgels were both found to exhibit capillary interactions, which appeared weaker for the more deformable hollow microgels. In line with our former study on anisotropic core-shell microgels, 33 it illustrates the importance of the anisotropy and softness on the interfacial assembly and may be employed to create defined nanostructured materials. In this case, more elongated objects with a more preferential side to side self-assembly, such as stretched core-shell microgels 32,53 might be interesting to create more defined assemblies.
We further demonstrate the influence of monolayer compression on the interfacial ordering. While both systems build up a preferential side to side ordering at larger compression, as revealed by the formation of small nematic domains, stiffer anisotropic core-shell microgels additionally tend to align parallel to the direction of the compression set by the barriers, whereas softer hollow microgels did not.
Both microgel monolayers are highly compressible allowing to finely tune the nearest neighbor distance between the microgels. Similar to spherical hollow microgels, hollow anisotropic microgels are for more compressible than their core-shell counterparts. We believe our results will allow to extent the use of microgel monolayers for nanolithography to more complex assemblies. Hereby, deposited monolayers of soft microgels were used as mask for the growth of nanowires to create photonic materials. Due to their compressibility, microgels have the advantage to allow the fine control of the nanowire dispacing. 54,55 The next challenge for anisotropic microgels is to promote their ordering into large nematic, smectic or even crystalline domains at the interface. More complex lattices may in the future be achieved that could polarize and guide the light, which further motivates the control of the domain orientation. Our results, as proofs of concept, assert the potential of anisotropic microgels for such applications but also the challenges it represents: first from the synthetic viewpoint to create highly monodisperse anisotropic microgels with a proper design and second in respect to their interfacial assembly related to their anisotropy and deformability to properly balance capillary interactions and allow long-range ordering at higher compression as well as domain alignment.

Acknowledgement
We gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft within SFB 985 "Functional Microgels and Microgel Systems". We thank Timon Kratzenberg for the help with the experiments on the Langmuir-Blodgett trough.

Supporting Information Available
The following files are available free of charge.
•     Figure S2 in the appendix.

Nearest neighbor distances
The development of the NND are illustrated as histograms in Figure S3 A-E for HS − 165 and in Figure S3

Computer simulations details
Size-limit effect. If we address to the collapse of a microgel in solution, the free energy of a single microgel can be written as a sum of two terms: S-6 The first one is the volume contribution (proportional to the volume V of the microgel); f being the free energy density and includes subchain elasticity, short-range interactions of polymer with the solvent, solvent entropy, etc. The second term is a surface free energy of the microgel in a poor solvent; γ being the surface tension coefficient. This term is proportional to the microgel surface area, S. The density f does not depend on the microgel size, however S/V is inversely proportional to the characteristic size of the microgel. In case of the spherical microgel S/V = 3/R, where R is the microgel radius. Such dependence means that the smaller the microgel, the higher the contribution of the surface tension to the total free energy. And vice versa: in the thermodynamic limit R → ∞ (the limit of the macroscopic gel), the surface does not play a role on the microgel collapse. In computer simulations, because of the limitation in the size of the simulation box (number of the beads), the characteristic sizes of the microgels are much smaller than in the experiments (tens of nanometers). Therefore, the modeling of the microgels, which have a size of hundreds of nanometers, requires a coarse-graining procedure leading to the effective reduction of gel size resulting in a recalculation of all the mechanical properties of the microgels. In the case of small microgels, the system becomes very sensitive to the disbalance between surface/volume terms, which is especially noticeable for oblong or hollow objects at the same coarse-grained level.
Method. The DPD uses a simplified description of the particles (coarse-grained approach). S4 In the DPD formalism, the bead is not just representing a single atom or a molecule of a real fluid, but rather a group of molecules localized in a certain volume and moving coherently.
The beads simply follow Newton's equations of particle motion: where m i , r i , and v i represent the mass, position vector, and velocity vector of a particle i, i ∈ (1..N ), respectively, N is the total number of particles, the superposed dot denotes a time derivative, the forces on the right side represent the conservative (subscript C), the dissipative (subscript D), and the random (subscript R) forces, respectively.
One of the most important parameters that can be tuned within a DPD simulation are the interaction parameters a ii and a ij describing repulsions between similar and dissimilar unbonded DPD beads, respectively. The parameters a ij are expressed in units of where r c is the cutoff distance, which defines the range of interaction between two beads.
There are several methodologies to choose it to describe phase separating fluids or fluid mixtures. S5-S8 Following the parametrization method employed by Groot and Warren S9 and Maiti & McGrother S5 , one could possible to represent a DPD particle as a single or multiple, N m water molecules. Based on the assumption of a ii = a jj , r c i = r c j and maintaining the constant density in the box, the DPD a ij parameters could be related to the Flory-Huggins theory: To preserve the compressibility, κ −1 , of water at different coarse-grained levels, a ij has to be related to the density, ρ DP D , within the simulation box and to the degree of coarse-graining, N m , as where α is considered to be a constant equal to 0.1 and independent of ρ DP D and a ii . We set the total number density of the system as ρ DP D = 3r −3 c . There are four different types of particles in our system: water (W), decane (D), the beads forming polymer NIPAM microgel shell (S) and solid core (C). Thus we have to set a ij where i and j ∈ (W, D, S, C) to establish the interactions between the different constituents.
Decan/water interface.To reproduce the decane/water mixture we set a ij in such a way to follow the experimental interfacial tension coefficient 52.33 mN/m. S10 We treated the decane as a single bead in the water/oil system similarly to Ref. S11 . In such representation the S-8 a W W = a DD = 25 k B T /r c . The interaction parameter between the liquids can be calculated based on the relation between the Flory-Huggins χ parameters and the activity coefficient of water. In the limit of infinite dilution: where γ ∞ i is the infinite dilution activity coefficient of water in decane and v ij is the ratio of the molecular volume of the decane v j to water v i , (i = W , j = D). The infinite dilution activity coefficient of water in decane as a function of temperature is presented in Table S2. Table S2: Infinite dilution activity coefficients ln γ ∞ W of water in decan S11 and interaction parameters a W D as a function of temperature over the range from 20 • C to 55 • C. The volume of water, v W = 25.58 Å 3 , and R 2 is the fitting correlation coefficient. The anisotropic microgels with elliptical solid core were designed as follows. Similarly to the work Ref. S2 , we constructed a unit cell of the diamond crystal lattice where the vertexes correspond to tetrafunctional crosslinkers. Then we assemble two cubic supercells S50 and S25 S-9 consisted of 50×50×50 and 25×25×25 unit cells, respectively. Supercell S50 is considered as a template for the solid nanoparticle, while S25 -as a template for the polymeric shell.
The dimensions of the hematite-silica core in the experiment were 330±12nm×75±8 nm.
We created the ellipsoidal nanoparticle having a similar ratio between the long and short semi-axis 4.4×1. The ellipsoidal core was constructed by inscribing the ellipsoid-shaped frame into S50 supercell and cropping all the beads, which are outside of the ellipsoid. To construct the polymeric shell around the solid core we used a scaled S25 supercell. All the bonds between the tetrafunctional crosslinker were replaced by the subchains of different lengths. The distribution of subchain lengths is described by a Gaussian distribution with a mean value equal to N and a standard deviation equal to dN . Two ellipsoidal frames of different radii were inscribed into the modified supercell. The small frame is necessary for the formation of the void in the polymeric network of the same sizes as the solid nanoparticle ones. The sizes of the large frame control the thickness and the shape of the polymeric shell.
All beads, which were located inside the small and outside of the large ellipsoidal frames, were cropped. The rest of the beads form microgel shell. The solid core was inserted into the void of the microgel with further grafting of the dangling chains of the polymeric shell to the nanoparticle surface. The dangling chains of the polymeric shell were physically attached to the nanoparticle surface. Going through all of the free ends of the dangling chains we monitor whether the distance between it and closest bead at the surface of the solid core satisfies the condition: |r end − r surf | ≤ r c . If so, a bond between the free end of the dangling chains and the bead of the surface of the solid core was formed. Only one bond could be formed between the free end and the solid core.
Since the real aspect ratio of the anisotropic microgel is unknown from the experiment, we designed three different microgels with the same shell mass and cross-linking density but different initial aspect ratios of the outer frame L/d = 2.8; 2.5 and 2.2 to capture the peculiarities of the shell dimensions ( Figure S5). The microgel, constructed using the outer frame with an aspect ratio of 2.8, is featured by a thicker shell at the top of the core compared S-10 to its side. The microgel, constructed using the outer frame with an aspect ratio of 2.2 has approximately the same thickness of the shell around the solid particle. The characteristics of the microgels used in simulations are presented in Table S3. Figure S5: The images of the models of the core-shell microgels with a different initial aspect ratio of the outer frame forming polymeric shell L f rame /l f rame = A)2.8, B)2.5 and C)2.2. To maintain the bonds between the beads in subchains of the microgels we applied a harmonic bond potential: where k bond ij = 35k B T /r 2 c , i, j ∈ (S, C) is the bond stiffness, and r eq = 0.6r c is the equilibrium bond length.
The number of monomers per subchain in real microgels is high enough. To simulate the macroscopic microgels, one has to use coarse-grained descriptions of the subchain. S12 Nikolov et al. S13 proposed a method to guide the relation between the number of monomers N and S-11 the flexibility of the subchains as well as the swelling degree of the microgel. In addition to the harmonic potential a bending angle potential as well as a segmented-repulsive potential (SRP) S14 between polymer beads has been introduced: where k bend ijk is the bending stiffness, and Θ is the angle between two pairs of connected beads sharing a common bead. r mid is the distance between the midpoint of two bonds of connected beads, r SRP = 0.5r c , the cutoff distance and C SS = 100k B T , the strength of the SRP potential between bonds. The SRP potential prevents polymer chains from crossing one another. The bending potential allows controlling the flexibility of the individual polymer subchains by changing the Kunh length. For k bend = 0, the Kuhn length is relatively short and is approximately equal to 0.6r c . The addition of bending stiffness leads to an increased Kuhn length and swelling degree of the microgel in a swollen state. We found that N = 6, dN = 1 and k bend = 5k B T are suitable to reproduce both the experimental swelling degree and deformability of the microgels.
The harmonic bond and angle potential had also to be set for the solid core to maintain its shape and volume and prevent its deformation at the interfaces.
r eq C = 0.8r c ; k bond CC = 100k B T /r 2 c , θ eq = 109.47 and k angle CCC = 100k B T . Without loss of generality, we set m = k B T = r c = 1. Consequently, our unit of time

Microgel characterization in bulk
The simulations of single microgels in bulk solution at different a SW values were performed.
Larger values of a SW correspond to bad solvents, whereas lower values of a SW correspond to good solvents. To identify the temperature responsiveness of the microgels at each a SW , we calculate hydrodinamic radii and L/d ratios for the microgels having different architecture.  The cross-sections of the equillibrated microgels of the thickness of d = 2r c taken through the center of mass are shown in Figure S7 (swollen state) and S10 (collapsed state). To estimate the actual radii of the microgel shell, solid particle or the void we have calculated the form factor. First, we started an approximation with a solid core. Fitting the data with the model of core-shell ellipsoids provides the dimensions, (19.4 ± 0.6)r c for the major and In Figure S7 the scattering data for the microgel shell of the CS and HS samples with different initial shell configuration in a swollen state: L f rame /d f rame = 2.8, 2.5 and 2.2 are presented.
We further tried to use the core-shell ellipsoid models to fit it. Nevertheless, in many cases S-14 we were not able to find the proper fit to describe the structure correctly. The fuzziness is needed to get into account to follow all details of the scattering points. To overcome it, we first used the LSQR approximation S2 of the inner and outer surface of the microgels shell to get the dimensions of the microgels. Relying on that data, we tried to find the best fit. The bright color solid lines on Figure S7 represent re-fitted form factors. The comparison of the sizes of the major L and minor axes l of the elliptical microgel and the cavity in a swollen state obtained in the simulations is presented in Figure S8. For the clarity we also plotted the fitted ellipsoids onto the cross-sections of the microgels in Figure S7. Figure S8: Sizes of the hollow (A) and the core-shell (B) anisotropic microgels in a swollen state (a SW = 25.6) obtained by the fitting of the form factors by means of the model of a spheroid ellipsoid particle with a core shell structure. L and d values of the major and minor axes of the elliptical cavity(gray), solid particles(black) and polymeric shell(colored) are shown as a cumulative bars. Numbers 2.8, 2.5 and 2.2 corresponds to the microgels with different initial shell configuration.
S-15 Figure S9: Shape anisotropy L/d of the hollow (dashed lines) and the core-shell (solid lines) microgels as a function of a SW .
We have started the quantitative analysis from the results for CS microgels. Obviously, the initial shell configuration determine the distribution of polymers surrounding the ellipsoidal core. After equilibration in a swollen state the ratio between thickness at the tip and the sides, δL/δd, are equal to 1.93, 1.51 and 1.1 for CS 2.8 , CS 2.5 and CS 2.2 samples respectively. The CS 2.2 can be regarded as a model for the microgel with constant thickness surrounding the ellipsoidal core, while the samples CS 2.5 and CS 2.8 -as model represented the microgels having slight and strong shell thickness anisotropy. Increasing the a SW lead to the collapse of the microgel shell accompanied by the increasing of the aspect ratio L/d, Figure S9, in agreement with the results obtained earlier. S15 The cross-sections of the equillibrated microgels in a collapse state above the VPTT are shown in Figure S10. When looking at the cross-sections of the microgels in the collapsed stated, one could find compact dense shell with a sharp surface. This is echoed by the shift of the first minima of the scattering curves to the right and the increase of the slope at high q values.
Independently of the sample etching of the core practically does not change the radii of the shells below VPTT which is in line with the results for the microgels with the thin shell. S15 Slight increase of the L/d value was observed for the sample HS 2.8 . Shell inhomogeneities S-16 at the tip of the microgel lead to the extra swelling of the shell, Figure S8. However, more noticeable changes occurred with the size of the cavity. After removal of the core L void > L core for the samples HS 2.5 and HS 2.2 . Increasing the a SW value leads to the collapse of the hollow microgels. Contrary to the CS microgels, collapse of the HS microgels does not lead to increase of aspect ratio L/d, Figure S9. In other words collapse of the microgel leads to a change in size, yet the anisotropic shape is conserved, confirming the conclusions in ref. S15 Moreover cavity was observed at all investigated a SW values. At the same time, the cavity of the microgels in the collapsed sate are much smaller comparing to the solid core. The collapse of the HS 2.8 leads to the formation of the cavity with a small asphericity value, while collapse of the HS 2.2 leads to a cavity with narrow elongated shape. Figure S10: Equilibrium structure of the swollen HS and CS microgels with different initial shell configuration: L f rame /d f rame = 2.8, 2.5 or 2.2 and their form factor profiles P(q). The cross-sections of the thickness of d = 2r c . Polymeric shell depicted in grey, solid nanoparticle -in black, water beads are not shown. a SW = 27.9. Symbols are the simulation data, and lines are their fitting curves.
S-17 Figure S11: Sizes of the hollow (A) and the core-shell (B) anisotropic microgels in a collapsed state (a SW = 27.9) obtained by the fitting of the form factors by means of the model of a spheroid ellipsoid particle with a core shell structure. L and d values of the major and minor axes of the elliptical cavity(gray), solid particles(black) and polymeric shell(colored) are shown as a cumulative bars. Numbers 2.8, 2.5 and 2.2 corresponds to the microgels with different initial shell configuration.

Interface deformation
The simulations of single microgels at the interface were performed. The estimation of the deformation of the water/decane interface were done. To identify the area of deformation, we constructed the surface mesh of the interface using OVITO tool. The procedure for surface construction from a set particles is described in Ref. S16 The radius of the probe sphere employed in the surface reconstruction algorithm was selected as 1. The smoothing level was equal to 100. To determine the surface area and plot the color height maps, the ParaView v.5.10.0 were used. S17 Figure S12: First and third rows: 3D view of the halves of the CS and HS microgels at the water/decane interface. Second and fourth rows: red and white and black gradient color code height maps of the interfaces. The interfaces are shown as smoothed surface mesh. Figure S13: First and third rows: Longitudinal cross-sections of CS and HS microgels. Solid particles are not shown. The decane molecules were plotted in red and black gradient color code accordingly to their z position with respect to the interface. Second and fourth rows: red and black gradient color code height maps of the interface. One part of the images is top view to the decane phase on which the outlines of the shell and the solid core are plotted. Another part is the smoothed water/decane surface mesh S-20