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Self-Healing Flexible Conductive Film by Repairing Defects via Flowable Liquid Metal Droplets

Self-healing flexible conductive films have been fabricated, evaluated, and applied. The film is composed of a fragile indium tin oxide (ITO) layer covered with sprayed liquid metal (LM) droplets. Self-healing of electrical conductivity is achieved via spontaneous capillary wicking of LM droplets into cracks/defects of the ITO film. The liquid metal adhering onto the ITO layer can also connect the ITO fragments during bending to keep the overall conductivity of the composite LM/ITO film stable. Stable and reversible electrowetting performance has been achieved with the composite LM/ITO as the conductive film, in either flat or curved states.


Substrate Cleaning Process before Deposition Liquid Metal
The surface cleaning process before any deposition is necessary for the reproducible results without any spurious side effects such as the prior dirt particles on the surface of the substrate [1]. In order to reduce the measurement error, we performed the same cleaning process on the substrate each time the substrate was prepared. The PET and ITO/PET substrates arrived with thin polymer films that protect these substrates from scratches and contamination during transportation and storage. The protective polymer film must be removed prior to deposition of the liquid metal. The cleaning process can remove the surface dirt generated by the substrate production process. The cleaning process involves ultrasonic cleaning for 10 minutes, and then rinsing with ethyl alcohol (CH3CH2OH, analytical grade) followed by rinsing with ultrapure (UP) water and subsequently drying in N2 (99.9%) gas flow.

Transmittance of LM Droplet Covered Films
In the present spraying method, small micro-and nano-droplets are deposited on substrates. Therefore, we speculate that such formed films may show certain optical properties, because the space among liquid metal droplets would provide some optical transmission, while the droplets are still connect with reserved electrical properties. Optical transmittance of these films were analyzed using an integrating spectrometer system (USB 2000, Ocean Optics, Orlando, Florida, USA). The transmittance data presented here are normalized to a reference substrate. Figure S1 depicts the optical transmittance of the films with various amount of LM. It is clear that the optical transmittance decreases when the LM amount increases for both single LM film and LM/ITO composite film, and reaches to the minimum transmittance of about zero when the surface is covered completely by continuous LM film.

Physicochemical Properties of Commonly Used Conductive Materials
Physicochemical properties of gallium-based liquid metal (LM) alloys are summarized in Table S1.

Property
Copper

Summary of the Properties of LM Film and LM/ITO Composite Film
As Table S2 shows, we summarize the performance of LM film and LM/ITO film with different film thicknesses in terms of transmittance, electrical conductivity, flexibility and electrowetting performance. It can be seen from Table S2 that when the thickness is small (λ < 4 μm), LM films and LM/ITO composite films have a certain transmittance, but the conductivity and flexibility of the LM films are poor in this thickness range, and the sheet resistance of the LM/ITO composite film is also large; when the thickness is large (λ > 4 μm), although the transmittance is greatly reduced, these two films have smaller sheet resistance, superior flexibility and better electrowetting properties. In practical applications, we can choose the appropriate film thickness based on different requirements for conductivity, light transmission, flexibility and electrowetting performance.  Figure S2 shows the photograph of the two-probe system for bending fatigue test. This system can keep on bending the samples from plane to specific curvature radius for customized cycles setting. The bending velocity was set at 1 cycle/6 s, and the bending radius of fatigue test was set to be 3.50 mm. With the increase of bending cycles, the sheet resistance of ITO film increases obviously to more than 104 Ω/sq; however, the sheet resistance of the composite LM/ITO film remains continuously at low values of less than 25 Ω/sq during the bending cycles. When the single ITO film was subject to bending fatigue loading, micro-cracks and defects were generated, and the measured sheet resistance exhibited degradation with increasing fatigue cycles as shown in Figure S3a. LM/ITO composite film is conductive even at small λ; therefore λ in the range of 1.31-11.46 μm were selected, showing typical sheet resistance of 0.04−30 Ω/sq ( Figure S3c). As shown in Figure S3b, when λ < 3.9 μm the sheet resistance of LM film is too high to be measurable since LM droplets form island structure (1) without formation of network (2) or continuous connection (3). Therefore, the samples with λ of 3.90-11.45 μm were thus selected to investigate the electrical conductivity of single LM films with typical sheet resistance of < 5.0 Ω/sq. A local magnification of 0-50 cycles is shown in Figure S3d for more details. Unlike single ITO film, the selected LM and LM/ITO films did not show a continuous increase in resistance during the folding process when λ reaches a threshold value of about 4.0 μm. For the single LM film, the film continuity and density is not guaranteed since the substrate itself is nonconductive. For the composite LM/ITO film, conductivity can always be ensured since the ITO film itself is conductive, and the measurements have proven it as well. Moreover, more stable electrical conductivity has been achieved for the composite LM/ITO films than that of single LM films according to the smaller variation in the ratio of the sheet resistances after and before the bending tests.

Surface Morphology of the Constructed Conductive Films
The surface morphologies of the conductive films were examined using an optical microscope (Lissview, L1600, GuangZhou, China) and a scanning electron microscope (SEM) system (ZEISS Ultra 55, Carl Zeiss, Oberkochen, Germany). Figure S4 shows the surfaces of single LM films and composite LM/ITO films at different film thickness after 2000 bending cycles. It can be seen that after bending, there are a lot of wrinkles on the surface of the LM film, and as for the LM/ITO composite film, cracks appear in the underlying ITO layer, and the liquid metal in the upper layer is wrinkled. Figure S5 shows the SEM images of the ITO cracks in the folded LM/ITO composite film, it can be seen that the liquid metal appears in the crack or cover the crack.

Connection Modes of LM and ITO in the LM/ITO Composite Films
In this work, the LM materials are assumed to be pure without contamination, and the thickness of LM film formed locally is uniform. When LM flows into cracks/crevices of the ITO film, LM and ITO are connected in-series. When LM is covered on the ITO layer as a continuous film, LM and ITO are connected in-parallel. The schematic drawing is presented in Figure S6. According to the two basic electric resistance Equations (1) and (2) as follows: ρ, S and L are the resistivity, cross-sectional area and length of the conductive materials, respectively. L1 and L2 are the film length along and perpendicular to the current flow direction, and d is the film thickness. The total resistance of the films connected in-series can be calculated using the Equation (3): The total resistance of the films connected in-parallel can be calculated using the Equation (4): (4) ρ1 is the resistivity of the Resistor 1 (LM, here), L1 and L2 are the length of LM and ITO films along the current direction, respectively, d1 is the thickness of LM, Rsq is the sheet resistance of ITO film, Lc is the sum of L1 and L2.
As seen from Equation (4), the total resistance of in-parallel connection is constant since the electric conductivity of LM material (3.4 × 10 6 S/m) is much higher than that of the ITO film (2 × 10 5 S/m). On the other hand, it means that the more in-series connection, the smaller the total resistance will be. Thus, when the LM film thickness is high enough to form a continuous film, or the bending induced film redistribution to connect discontinuous LM droplets of segments to a continuous film, the sheet resistance becomes stable.

Characterization of the Electrowetting Performance on the Flexible Films
The mechanism of self-healing conductivity of LM/ITO films on PET substrates has been achieved via LM droplets filling into an ITO crack by capillary wicking and connect adjacent cracks with in-series or in-parallel. To evaluate the reliability and applicability of the constructed flexible films, the multilayer EWOD device was built by stacking a flexible liquid-infused-film (LIF) on top of a conductive film. The combination of two fluidic materials of liquid metal and silicone oil in the LM/ITO conductive and LIF hydrophobic insulating layer, respectively, show excellent flexibility and compatibility. The EWOD devices show smooth surfaces without gas bubbles. Contact angles were measured using an OCA 15 Pro contact-angle system (Dataphysics, Germany). Voltages were applied via two tungsten wires with one end connected to a power source (PSW, China) and the other end connected to the droplet or conductive film. Figure S7 is a scanning electron microscope (SEM) image of the PTFE membrane for constructing the LIF, representing the porous structure for liquid filling possibility. The average pore size and thickness of the PTFE membrane are ~200 nm and ~20 μm, respectively.  Figure S8 presents corresponding contact angles of sessile drops on different films driven by electric fields. Figure S8a shows that, with the increase of LM film thickness of the single-LM films, the contact angle difference increases from 27 to 60° driven at 450 V. Figure S8b demonstrate the stable electrowetting performance which is not obviously affected by the LM film thickness for the composite LM/ITO films. The comparison shown in Figure S8c is consistent with previously presented results in this work. On single-LM films, the electrowetting performance was only preserved when the LM thickness is >4 μm, as shown in Figure S9a. In Figure S9b, reversibly electrowetting performance was observed on all LM/ITO films as conductive layers.  Figure S10 shows a microscopic image of an Ag-nanowire based conductive film. Although the whole film is conductive, most area is still empty according to the network structure. Thus, only the contact area of the water droplet and the conductive wires works to drive the droplet. As a result, only slight contact angle change of about 10° (decreases from 110 to 101° before saturation) could be achieved using such a network conductive film to drive a sessile droplet via electrowetting mechanism.