Dynamic Contact Angle at the Nanoscale: A Unified View

Abstract

Generation of a dynamic contact angle in the course of wetting is a fundamental phenomenon of nature. Dynamic wetting processes have a direct impact on flows at the nanoscale, and therefore, understanding them is exceptionally important to emerging technologies. Here, we reveal the microscopic mechanism of dynamic contact angle generation. It has been demonstrated using large-scale molecular dynamics simulations of bead-spring model fluids that the main cause of local contact angle variations is the distribution of microscopic force acting at the contact line region. We were able to retrieve this elusive force with high accuracy. It has been directly established that the force distribution can be solely predicted on the basis of a general friction law for liquid flow at solid surfaces by Thompson and Troian. The relationship with the friction law provides both an explanation of the phenomenon of dynamic contact angle and a methodology for future predictions. The mechanism is intrinsically microscopic, universal, and irreducible and is applicable to a wide range of problems associated with wetting phenomena.

Hydrodynamics of Moving Contact Lines: Macroscopic versus Microscopic

“…We have clarified this issue previously, as other groups did, 45 by directly probing the Young-Dupré equation in equilibrium conditions by placing a cylindrical (to avoid possible line tension effects) drop of 40000 particles on the substrate. 39 The static contact angle was then obtained in two ways: first by direct measurements from the free surface profiles using the same methodology as described above and second, for comparison, via the Young-Dupré equation using independently calculated equilibrium surface tensions. We have found a very good agreement between the two angles, when the highly bend region was excluded from the contact angle evaluation procedure, in full accordance with the fact that the contact angle is an experimentally observed, macroscopic quantity.…”

“…We would like to note that the particular set of MDS parameters used and discussed in this work is representative of a larger set of MDS simulations produced in the previous work. 39 The simulations were performed for different model liquids with N B ranging from N B = 1 to N B = 30, at different static contact angles θ 0 in the range 0 • ≤ θ 0 ≤ 106 • , at different system sizes H, Figure 2 and Figure 3, ranging from H = 40 σ ff to H = 100 σ ff , at different contact line velocities 0.005 u 0 < U ≤ 0.2 u 0 (u 0 = ff /m f ) and at different temperatures 0.8 ff /k B ≤ T 0 ≤ 1 ff /k B , see details in the previous work. 39 While the value of the macroscopic model parameters, such as Re, Ca, ρ…”

“…39 The origin of that singular force F has been studied in detail using MDS. 39 It has been shown that the force is the consequence of the microscopic processes taking place at the contact line on the length scale of a few atomic distances, which is induced by the interaction potential of the constituent molecules. The observed length scale defines the size of the contact line zone.…”

Characterizing the bifurcating configuration of hydrogen bonding network in interfacial liquid water and its adhesion on solid surfaces

“…15,16 Above the depletion region, wave-like spatial undulations of liquid density have been reported, [17][18][19] implying a new mechanism of energy dissipation in the form of layer-by-layer friction during the process of dynamic wetting. [20][21][22] It was also reported that such spatial undulations can break the ideal tetrahedron geometry of hydrogen bonds, 23 which is usually formed in bulk water, and may give rise to the polarity of interfacial water. 24,25 In this respect, the identication of truly interfacial molecules (ITIM) analyses revealed that the effect of structural change on interfacial properties may be limited to the close vicinity of the interface, i.e., the rst molecular layer.…”

Modified smoothed particle hydrodynamics approach for modelling dynamic contact angle hysteresis

“…Experimental investigations on dynamic contact angle could be strongly influenced by small-scale physical and chemical heterogeneities, impurities adsorbed on the solid surface, growth and dissolution of bubbles, etc [24]. Therefore, considering the above-mentioned limitations and availability of experimental conditions and facilities, the numerical approaches, including molecular dynamics (MD) [11,25,13], lattice-Boltzman methods (LBM) [10,26,27] and SPH [12,14], serve as powerful tools to study the contact angle dynamics and the fundamental mechanisms therein. At the micro-scale, Koplik et al [11] identified rate-dependent behaviour for dynamic receding angle with MD simulation in an immiscible two-fluid system.…”