Moving contact line

While the above refers mainly to the static limit, new effects come into play when a moving contact line, i.e. spreading, is considered. It has been observed experimentally that the contact angle of a moving contact line 0, the dynamic contact angle, deviates from the corresponding static value 0. As an example, for a completely wettable surface (i.e. 6(, = 0), a relationship of the form... 

Kralchevsky P, Danov KD, Kolev VL, Gurkov VD, Temelska ML, Brenn G (2005) Detachment of oil drops from solid surfaces in surfactant solutions molecular mechanisms at a moving contact line. Indust Eng Chem Res 44 1309-1321... 

Davis SH. (1980) Moving contact lines and rivulet instabilities. Part 1. The static rivulet. J Fluid Mech 98 225-242. 

Liquid viscosities have been observed to increase, decrease, and remain constant in microfluidic devices as compared to viscosities in larger systems. ° Deviations from the no-slip boundary condition have been observed to occur at high shear rates. One important deviation from no-slip conditions occurs at moving contact lines, such as when capillary forces pull a liquid into a hydrophilic channel. The point at which the gas, liquid, and solid phases move along the channel wall is in violation of the no-slip boundary condition. Ho and Tai review discrepancies between classical Stokes flow theory and observations of flow in microchannels. No adequate theory is yet available to explain these deviations from classical behavior. ... 

Figure 2—11. A sketch of the moving contact line problem. In (a) the interface is assumed to he moving with velocity —Ualong the solid wall. In (b), the equivalent problem is shown in which the interface is viewed as fixed and the wall moving in its own plan with velocity U.

There is also the possibility of slip at a fluid-fluid interface, especially for a pair of thermodynamically incompatible fluids. However, we are not aware of any evidence of slip at an interface between two small-molecule Newtonian fluids. One reason is that there are no examples of flow conditions analogous to the moving contact line at solid boundaries that can lead to very large tangential stress. Hence, because p is usually very small for Newtonian fluids, conditions that would produce any significant slip are absent, even assuming that... 

This may seem surprising if we stop and think a little about the assumptions of the analysis. The theory is based on the thin-film model, in which the flow is nearly a unidirectional flow. The shape function f(rj) is plotted in Fig. 6-4. Evidently the assumption of nearly unidirectional flow must break down totally at the front of the spreading drop. Furthermore, we have completely neglected to say anything about the moving contact line where the drop meets the solid substrate. In spite of these apparent flaws in the theory, the... 

After the static test mentioned above, the method is now tested for the impact and spreading of a glycerin droplet on a wax substrate and the computational results are compared with the experimental data of Sikalo et al. [32], The details of the experimental setup, material properties and computational model can be found in Refs. [33, 51]. The computed and experimental spread factor and contact line are plotted in Figs. 19a and b, respectively. These figures show that the present front-tracking method is a viable tool for simulation of interfacial flows involving moving contact lines. 

MACROSCOPIC AND MESOPHYSICS TOGETHER THE MOVING CONTACT LINE PROBLEM REVISITED... 

Keywords Moving contact line, fluid mechanics, viscous fluid, mesoscopic effects... 

This moving contact line problem has received a lot of attention over the years [2], Recently there has been significant progress in this problem, both in experiments and theory. The experiments have clearly shown what could be called a dynamical wetting transition. This was already in the work of Blake... 

Macroscopic and mesophysics together the moving contact line problem revisited Yves Pomeau References... 

Numerical simulations have been made possible by the availability of experimental data on a well-characterized geometry and with accurate concentration measurementst The simple geometry of ceramic monoliths is essential for accurate numerical modelling with no independent adjustable parameters such as tortuosities or effective diffusions within porous media. The only adjustable parameter, the peak diffusion at the moving contact line, will be eliminated when an acceptable, simple flow model of split-ejection streamlines is available. 

Contact Angles and Flow Patterns near the Moving Contact Line... 

Figure 10.4 Schematic representation of flow patterns near a moving contact line during immersion of a solid substrate into a pool of liquid, (a) Split-injection streamline in phase B and rolling pattern in phase A. (b) Transition flow pattern with motionless interface and rolling motion in phases A and B. (c) Rolling motion in phase B and split-ejection streamline in phase A...

The dependence of dynamic contact angles on moving contact line velocity has been the subject of many experimental studies since this is an important parameter in highspeed coating. For an early review of experimental data on dynamic contact angles, refer to Dussan (1979). For more recent reviews on dynamic contact angles, refer to the excellent contributions of Blake (Chapter 5) and Kistler (Chapter 6) in Berg (1993). 

For very small dynamic contact angles, the liquid is not completely removed by the split streamline and it is entrained between the film and the solid surface, creating what is known as a wet LB film. Water trapped between the solid surface and the LB monolayer prevents adhesion and is a leading cause of monolayer instability. Petrov etal. (1980) sketched the flow pattern near the moving contact line. The flow pattern is the one described here for region IV. The authors, however, reference Huh and Scriven (1971)... 

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