Moving mechanisms of the three-phase contact line in a water–decane–silica system

Abstract

The movement of the three-phase contact line with chain molecules in the liquid phase displays more complex mechanisms compared to those in the usual liquid-liquid-solid systems and even to the gasliquid-solid systems controlled by the traditional single-molecule adsorption-desorption mechanisms.By introducing decane molecules with chain structures, we demonstrate from molecular dynamics insights that the moving mechanism of the contact line in a water-decane-silica system is totally different from traditional mechanisms. Three different wettability-related moving mechanisms including "Roll up", "Piston" and "Shear" are revealed corresponding to the hydrophilic, intermediate and hydrophobic three-phase wettability, respectively. In the "Roll up" mechanism, the decane molecules are rolled up by the competitively adsorbed water molecules and then move forward under the driving force; when the "Piston" mechanism happens, the decane molecules are pushed by the piston-like water phase owing to the comparable adsorption interactions of the two liquids on the solid surface; in the "Shear" mechanism, the contact line is hard to drive due to the stronger decane-silica interactions but the decane molecules far away from the solid surface will move forward. Besides, the time-averaged velocity of the moving contact line is greatly related to the moving mechanisms. For the "Roll up" mechanism, the contact line velocity increases first and then reaches a steady value; for the "Piston" mechanism, the contact line velocity has a maximum value at the start-up stage and then decreases to a stable value; for the "Shear" mechanism, the contact line velocity fluctuates around zero due to the thermal fluctuation of the molecules. Additionally, the mean distance from Molecular Kinetics Theory increases with decreasing hydrophilicity and the displacement frequency in "Roll up" mechanism is 2 orders of magnitude higher than that in the "Piston" mechanism, further demonstrating the different moving mechanisms from a quantitative point of view. Fig. 7 MKT parameters. (a) Velocity plotted in logarithmic scale versus cos q d . (b) Parameters l and K 0 of the MKT at different three-phase wettabilities and the comparison with other works.This journal is

Effects of Molecular Chain Length on the Contact Line Movement in Water/n-Alkane/Solid Systems

“…Here, the time-variation velocity decreases with increasing the molecular chain length. However, the trend that the velocity increases firstly and then reaches to a steady value (with weak fluctuation due to the molecular movement at nano-scale) is almost the same at different chain length, indicating that the moving mechanism of "Roll up" is the same at all of these cases [14].…”

“…The displacing side contains 16,245 molecules of H 2 O with transferable intermolecular potential with 3 points (TIP3P) model. Simulations are performed using periodic boundary conditions in the directions parallel to the silica surface, and using reflective boundary conditions in the directions vertical to the silica surface [14]. For the α-quartz surface, the non-bridging oxygen atoms which are bonded to only one Si atom are attached to the hydrogen atoms [25].…”

“…Then a 6 nm × 6.4 nm wall is added from the left of the displacing side to drive the movement ( Figure 1). We previously described the simulation procedures [14]. An external force F is added to drive the systems, then simulations are performed to reach a steady state in the systems.…”

“…We have found that the moving contact line has several typical mechanisms at hydrophilic, intermediate, and hydrophobic systems. Noteworthy, in the hydrophobic system the contact line is hard to be driven [14]. Thus, here we only consider two typical three-phase wettabilities (hydrophilic/45 • and intermediate/90 • ) by changing the energy parameter ε in Equation (1).…”

“…We showed previously that the moving mechanisms of the three-phase contact line in a liquid-liquid-solid system were greatly related to the three-phase wettability. In systems with different mechanisms, the trends of the time-variation velocity of the contact line and the parameters of the molecular-kinetics theory showed great differences [14].…”