{'fraction_non_alphanumeric': 0.03174083769633508, 'fraction_numerical': 0.007806656694091249, 'mean_word_length': 4.810157523085279, 'pattern_counts': {'":': 0, '<': 4, '<?xml version=': 0, '>': 1, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 1, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Kinetic Slip Condition, van der Waals Forces, and Dynamic Contact Angle

The profiles of a spreading wetting film are computed taking into account intermolecular forces and introducing a kinetic slip condition at a molecular cut-off distance. This eliminates the stress singularity, so that both"true"and"visible"contact angles are defined unequivocally. The"true"contact angle at the cut-off distance depends on the slip length as well as on the edge propagation speed, but not on gravity or asymptotic inclination angle. These macroscopic factors influence, however, the"visible"contact angle observed in the interval where the actual film profile departs from the intermediate asymptotic curve.


I. INTRODUCTION
The two basic unsolved problems in the theory of a moving three-phase contact line are defining the contact angle and resolving the infamous viscous stress singularity. Different approaches to both problems, neither of them satisfactory, have been reviewed by Shikhmurzaev [1]. Even under equilibrium conditions, the structure of the three-phase region cannot be understood without taking into account intermolecular interactions between the fluid and the solid support [2][3][4]. It becomes apparent that motion of a contact line is an intrinsically mesoscopic problem, and the dynamical theory should blend factors contributed by hydrodynamics and physical kinetics.
The "standard" equilibrium contact angle θ e is defined by the Young-Laplace formula which involves surface tension σ of an interface between two semi-infinite fluid phases (in the simplest case, a one-component liquid and its vapor) and (non-measurable) surface tensions between the solid support and either fluid, σ l and σ v . Since, by definition, the standard surface tension refers to a boundary between semi-infinite phases, the surface properties should be modified when the three-phase region falls within the range of intermolecular forces, and therefore the classical formula is likely to fail in a close vicinity of the contact line. This region is too small to be detected by available measurement techniques, but modification of interfacial properties is often revealed by the formation of a precursor film.
Thus, even under equilibrium conditions the contact angle, generally, varies with the distance from the contact line and cannot be defined unequivocally.
In a dynamical situation, such as wetting, spreading or draw-down of a meniscus, the interfacial curvature, and hence, the change of the contact angle, are further influenced by the viscous stress. A properly defined contact angle fixes the boundary condition at the edge of an advancing or receding film and is therefore a necessary ingredient for computation of macroscopic flows, influenced also by external forces, such as gravity, and by changes in temperature and chemical composition through buoyancy and Marangoni effect. Macroscopic measurements yield the so-called "visible" contact angle, differing from both the "standard" value in Eq. (1) and a hypothetical "true" (microscopic) value. Both "true" and visible contact angles should depend on the flow velocity and are subject to hysteresis.
One should be warned that the very notion of a "true" interfacial angle is precarious, since it extrapolates the concept of a sharp interface of a continuous theory to molecular distances.
This notion is eliminated altogether in molecular simulations [5,6] and in diffuse interface theories [7][8][9][10]. In continuum theories incorporating intermolecular forces the "true" contact angle can be defined at most at the molecular cut-off distance d. This is sometimes forgotten when hydrodynamic theory leads to the appearance of unphysically narrow boundary layers.
Bearing in mind limitations of continuum mechanics extended to molecular scales, we attempt in this communication to combine the standard hydrodynamic theory with a simple kinetic description of sliding motion in the first molecular layer adjacent to the solid support.
The thickness of the sliding layer is identified with the cut-off length in the van der Waals interaction potential; thus, the theory is expected to operate at about the same crude level as the classroom derivation of the van der Waals equation of state [11].
The paper is organized as follows. We start in Section II with a detailed discussion of the slip condition. Basic equations in lubrication approximation are formulated in Section III. Intermediate asymptotics of the solutions at relatively short macroscopic distances from the contact line, where gravity still does not come into play, are discussed in Section IV.
Solutions describing the form of a stationary meniscus on a moving inclined plane are given in Section V.

II. SLIP CONDITION
With any finite contact angle given as a boundary condition, a moving contact line still cannot be described within the framework of conventional hydrodynamics, since the classical no-slip condition on a solid substrate generates a multivalued velocity and, hence, an infinite stress in the vicinity of a contact line, leading formally to an infinite drag force [12,13].
The most common way to eliminate the viscous stress singularity is to impose a phenomenological slip condition. Presence of slip at a microscopic scale comparable with intermolecular distances is an established fact in Maxwell's [14] kinetic theory of gases; for dense fluids it is a feasible hypothesis supported by molecular dynamics simulations [5,6]. The two alternatives are slip conditions of "hydrodynamic" and "kinetic" type.
The version of the slip condition most commonly used in fluid-mechanical theory is a linear relation between the velocity component along the solid surface u s and the shear stress [15,16]. The proportionality constant contains a phenomenological parameter -slip length -characterizing intermolecular interaction between the fluid and the solid; in liquids this length should be small, so that the effect of sliding becomes significant only in the vicinity of a moving contact line where stresses are very large. This condition has been widely used for modeling macroscopic flows involving the contact line motion [17][18][19][20][21]. It does not eliminate the stress singularity but only makes it integrable, thus leaving a logarithmic (integrable) singularity of the interfacial curvature. This leads formally to a breakdown of the commonly used lubrication approximation in the vicinity of a contact line that can be remedied only by further ad hoc assumptions, making the slip length dependent on the distance from the contact line [18,21].
This drawback may be mere technical, but a more serious disadvantage of hydrodynamic slip theories lies in their inherent inability to predict the dynamic contact angle. Thus, the two basic problems become disentangled, and, in addition to a phenomenological slip coefficient, empirical relationships between the velocity and contact angle have to be introduced in model computations.
Another version of the slip condition, rooted in physical kinetics [22][23][24][25], defines the slip velocity through the gradient of thermodynamic potential w along the solid surface: where D is surface diffusivity, n is particle number density, k is Boltzmann constant, T is temperature, and ∇ is two-dimensional gradient operator along the solid surface. The condition (2) follows rather naturally from considering activated diffusion in the first molecular layer adjacent to the solid. In contrast to the hydrodynamic slip condition, the kinetic condition (2) can be used to define the "true" dynamic contact angle at the contact line in a unique way, as we shall see below.
Extrapolating the continuous description of fluid motion to a molecular scale might be conceptually difficult but unavoidable as far as interfacial dynamics is concerned. Longrange intermolecular interactions, such as London-van der Waals forces, still operate on a mesoscopic scale where continuous theory is justified, but they should be bounded by an inner cut-off d of atomic dimensions. Thus, distinguishing the first molecular layer from the bulk fluid becomes necessary even in equilibrium theory. In dynamic theory, the motion in the first molecular layer can be described by Eq. (2), whereas the bulk fluid obeys hydrodynamic equations supplemented by the action of intermolecular forces. Equation (2)  Miller and Ruckenstein [26] used the dependence of the disjoining pressure generated by London-van der Waals forces in a wedge to compute the "true" equilibrium contact angle. This result has been used by Hocking [27] to set the boundary condition at the contact line in the hydrodynamic theory, and by Ruckenstein and Dunn [23] to compute the slip velocity. Order-of-magnitude estimates show, however, that at small inclination angles necessary to justify the lubrication approximation used in hydrodynamic theory the correction to disjoining pressure due to surface inclination is extremely small, and the "true" angle may be formally attained only at distances far below atomic dimensions. At higher inclination angles, the computation fails technically, since the interface must be curved, and its form should be determined by a very complicated integro-differential equation involving intermolecular interactions as well as viscous stress and surface tension.
We propose to use the kinetic slip condition in the another way, to obtain a relation between the slip velocity and the thermodynamic potential at the contact line by considering the motion at the point where the film thins down to the minimal thickness h = d. This is the advancing edge of a wetting film, or a retreating edge of a dewetting film, dividing it from the dry solid surface. If the fluid is not volatile, the motion at this point should be pure slip, while standard caterpillar motion is retained at observable macroscopic distances.
In the case of an advancing wetting film, we expect that the leading edge is followed by a thin precursor film where surface tension is negligible and the action of intermolecular forces driving the advancing film is balanced by viscous dissipation. The boundary condition at the leading edge will be then the same Eq. (2) with u s replaced by the edge propagation speed U and ∇w computed at h = d with surface tension neglected.
Another possibility might be to assume that the slip motion at the edge is driven by the potential drop over the molecular cut-off distance d. This yields, by analogy with Eq. (2), the boundary condition at the contact line where w(d) is the thermodynamic potential of the film of the minimal thickness; the potential at the dry surface is taken as zero. After w(d) is computed as in the following Section, Eq. (3) turns into a condition relating the curvature at the contact line with the propagation speed.
We shall see that this condition leads in fact to non-physical results at small propagation velocities. At large velocities, computations using the alternative boundary conditions yield practically the same results (see Section IV).

III. BASIC EQUATIONS AND SCALING
We shall use the lubrication approximation, which is formally obtained by scaling the two-dimensional gradient operator along the solid surface ∇ ∝ ǫ, ǫ ≪ 1. Respectively, time is scaled as ∂ t ∝ ǫ 2 , the velocity in the direction parallel to the solid support as u ∝ ǫ and transverse velocity as v ∝ ǫ 2 . This implies that the thermodynamic potential w is constant across the layer. The gradient of w in the direction parallel to the solid support serves as the forcing term in the Stokes equation. The velocity profile u(z) across the film verifies where η is the dynamic viscosity. We use here the no-stress boundary condition on the free surface z = h, but replace the usual no-slip boundary condition on the solid support u(0) = 0 by the slip condition at the molecular cut-off distance d with u s given by Eq. (2).
The solution in the bulk layer d < z < h is where λ = Dη/nkT is the effective slip length.
The general balance equation for the film thickness h, obtained from the kinematic condition on the free surface, can be presented as a generalized Cahn-Hilliard equation, where the two-dimensional flux j in the plane aligned with the solid support is proportional to the two-dimensional gradient of the potential w: The effective mobility η −1 Q(h) is obtained by integrating Eq. (5) across the layer. Including also the constant slip velocity u = −λ 2 η −1 ∇w in the slip layer 0 < z < d, we have Since both λ and d are measurable on the molecular scale (see the estimates in the end of this Section), this expression does not differ in a macroscopically thick layer from the standard shallow water mobility Q 0 = 1 3 h 3 , and the correction becomes significant only in the immediate vicinity of the contact line.
The potential w is computed at the free surface z = h. Taking into account surface tension, gravity, and van der Waals force, it is expressed as where A is the Hamaker constant, g is acceleration of gravity, ρ is density, σ is surface tension, and ǫα is the inclination angle of the solid surface along the x axis. The dummy small parameter ǫ is the ratio of characteristic scales across and along the layer; the relative scaling of different terms in Eq. (8) is formally consistent when σ = O(ǫ −2 ). Further on, we suppress the dependence on the second coordinate in the plane, replacing the Laplacian by In the following, we shall consider the film with a contact line steadily advancing along the x axis in the negative direction with the speed U. Then Eq. (6) can be rewritten in the comoving frame, thus replacing h t by Uh x , and integrated once. Making use of the condition of zero flux through the contact line to removing the integration constant yields This equation can be further transformed using h as the independent variable and y(h) = h 2 x as the dependent variable. We rewrite the transformed equation introducing the capillary number Ca = |U|η/σǫ 2 , van der Waals length a = ǫ −1 (|A|/6πσ) 1/2 , and gravity length The boundary condition following from (2) The alternative boundary condition (3), set at h = d, is rewritten, using Eq. (8) and neglecting the gravity term, as Equation (10) contains three microscopic scales d, a, λ and a macroscopic gravity length b. The natural choice for d is the nominal molecular diameter, identified with the cut-off distance in the van der Waals theory. The standard value [3] is 0.165nm. The slip length is likely to be of the same order of magnitude. The approximate relation between viscosity η and self-diffusivity D m in a liquid [28] yields D m η ≈ 10 2 kT /3πd. The surface diffusivity should be somewhat lower than the diffusivity in the bulk liquid, and with D/D m ≈ 0.1 we The van der Waals length a depends on the relative strength of liquid-liquid and liquidsolid interactions. The Hamaker constant for the pair fluid-solid is defined [3] as where C s , C f are constants in the long-range attraction potential C/r 6 , respectively, for the pairs of fluid-solid and fluid-fluid atoms removed at the distance r; n s is the solid number density; The effective interaction parameter C is defined by the above identity. We shall assume A > 0, which corresponds to the case of complete wetting. The estimate for surface

IV. INTERMEDIATE ASYMPTOTICS
In the intermediate region, where h far exceeds the microscopic scales d, a, λ but is still far less than the capillary length b, the film profile is determined by the balance between viscous stress and surface tension. The asymptotics of the truncated Eq. (10) (with d, a, λ, where h 0 is an indefinite constant. The first term of this asymptotic expression has been obtained by Hervet and de Gennes [29], who have also reported the value of h 0 . This constant can be obtained by integrating Eq. (10) (with gravity neglected) starting from the boundary condition (11) or (12) and adjusting another necessary boundary value to avoid runaway to ±∞. There is a unique heteroclinic trajectory approaching the asymptotics (14). It is very sensitive to the initial conditions as well as to the molecular-scale factors operating close to the contact line. The growth of the inclination angle is never saturated, as long as macroscopic factors (gravity or volume constraint) are not taken into account.
Equation (10) can be integrated using the shooting method: either starting from the boundary condition (11) and adjusting y ′ (d) or starting from the boundary condition (12) and adjusting y(d) to arrive at the required asymptotics at h → ∞. Further on, we will measure all lengths in the molecular units and set d to unity. The solution in the intermediate region depends on the physical parameters a, λ as well as on the capillary number Ca that includes the propagation speed U. The latter's impact is most interesting for our purpose.
Examples of the computed dependence of the inclination angle θ = y(h) on the local film thickness h using the boundary condition (11) at different values of Ca are given in Fig. 1.
The curves using the boundary condition (12) in Fig. 1b, show peculiar (apparently, nonphysical) reversal of the dependence of the inclination angle on Ca, resulting in an increase of the "true" contact angle θ(d) with decreasing velocity. Indeed, at U → 0 this condition yields a spurious balance between intermolecular forces and surface tension leading to an unstable stationary state, similar to the erroneous inference of a wetting film with the right contact angle in Ref. [30] discussed in our earlier paper [31]. The anomaly, however, quickly disappears at observable distances.
The curve segments at h ≫ 1 can be fit to the asymptotic formula (14) to obtain the integration constant h 0 . It should be noted that the asymptotic formula (14) can be used only when h is logarithmically large, and the convergence, as estimated by the second term, is slow; therefore h 0 can be only obtained approximately from the computed profiles. The dependence of h 0 on Ca based on Fig. 1a is shown in Fig. 2. We see here a rather strong variation of the integration constant, unlike a single "universal" value reported in Ref. [29].

V. DRAW-DOWN OF A MENISCUS
The simplest stationary arrangement including gravity is realized when an inclined plane, dry at x → −∞ slides in the direction of a wetting layer. Solving Eq. (10) with the same boundary condition (12) as before brings now to the asymptotics y = √ α at h → ∞ that corresponds to a horizontal layer.
The curves y(h) seen in Fig. 3a and Fig. 3c all depart from the intermediate asymptotic curve obtained for infinite b as in the preceding Section. However, due to extreme sensitivity of the shooting method to the choice of the missing initial value, one has to integrate from the outset the full equation rather than trying to start integration from some point on the intermediate asymptotic curve. One can see that the maximum inclination angle (which may be identified with the "visible" contact angle) grows as b decreases. This increase is, however, not pronounced when the initial incline (identified with the "true" contact angle) is high.
One can distinguish therefore between two possibilities: first, when the main dissipation is due to kinetic resistance in the first monomolecular layer that raises y(d), and second when the viscous dissipation prevails and the inclination angle keeps growing in the region of bulk flow. Take note that even in the latter case the region where the inclination and curvature are high are close to the contact line when measured on a macroscopic scale. to the data of Fig. 3b. The points of the upper curve are computed in a similar way using the boundary condition (12). The first result appears to be more physically reasonable, since the angle drops close to zero at small flow velocities, while in the alternative computation it remains finite (see also the discussion in the preceding Section). The proportionality of the inclination angle to Ca 1/3 (which leads to the well-known Tanners law of spreading [4]) is a property of the intermediate asymptotics (14) that can be deduced from scaling [31], although the universality is slightly impaired by the dependence of the integration constant h 0 on velocity seen in Fig. 2.. The one-third law is inherited by the dependence θ m (Ca), since the inclination angle reaches its maximum while the gravity-dependent profile is still close to the intermediate asymptotic curve. be solved by shooting method with reasonable accuracy in a very wide range extending from molecular to macroscopic scales, and the "true" contact angle at the cut-off distance can be defined unequivocally.
The "true" angle (unobservable by available techniques) depends on the slip length as well as on the edge propagation speed, but not on gravity or asymptotic inclination angle.
These macroscopic factors influence, however, the "visible" contact angle observed in the interval where the actual film profile departs from the intermediate asymptotic curve. Since the latters location, though not shape, depends on the molecular-scale factors, as well as on the cut-off distance, the visible angle depends on both molecular and macroscopic factors.
Thus, the lack of simple recipes for predicting the value of dynamic contact angle is deeply rooted in the mesoscopic character of the contact line.