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Parametric Study of Liquid Contact Line Dynamics: Adhesion vs. Hydrodynamics


There are tremendous interests regarding the wettability of solid surfaces and controlling the wettability on the solid surfaces in industry, technology such as efficiency of oil recovery, micro-fluidics and nano-fluidics, drag reduction on airplane wings, efficient power plants and many other applications. To resolve such challenges, it is required to enhance the knowledge more deeply to understand the underlying physics of fluid/solid interaction at the liquid contact line that describes the dynamics of spreading.

Hence, in this research, experimental techniques have been performed to investigate parametric study about the dynamics of the liquid contact line (i.e. three-phase contact line) with consideration on molecular-kinetic theory (which concentrates on the adhesion of molecules at the vicinity of the liquid contact line) and the hydrodynamics theory (which focuses on the bulk motion of the liquid on the solid surface).

Over half a century, there have been many experimental/numerical investigations of the moving contact line in forced and spontaneous spreading. Surprisingly, there have been no experimental studies comparing these for the same solid/liquid/vapor system. In the present research such experiments have been performed on identical liquid-solid systems. It has been found out that there exists a huge distinction between experimental results obtained from spontaneous and forced spreading. For spontaneous spreading, excellent agreement has been found using the hydrodynamics theory, as expected. For forced spreading it was shown that hydrodynamic theory does not apply, but instead the molecular-kinetic theory does. This distinction between spontaneous and forced systems has never been noted before. Moreover, this research has provided a hypothesis for predicting the more appropriate model to describe the contact line dynamics for spontaneous and forced systems.

The forced spreading dynamics on low-energy surfaces (i.e. hydrophobic, ultra-hydrophobic glass surfaces, and micro-textured Teflon surfaces) has been investigated. The dynamics of spreading of several Polyethylene Glycol/Water (PEG/water) mixtures, with different weight ratios signifying the effect of the viscous force on spreading, on Teflon substrates (i.e. hydrophobic surfaces), ultra-hydrophobic sprayed glass substrates, and micro-textured Teflon plates have been investigated using Wilhelmy plate method with Tensiometer. It has been found out that spreading dynamics of the PEG/water mixtures on hydrophobic surfaces, ultra-hydrophobic glass surfaces, and micro-textured Teflon substrates are described more appropriately with molecular-kinetic theory.

The wettability of emulsions is a prominent factor with a broad impact in an extensive variety of industrial applications ranging from the petroleum to cosmetic industries. Surprisingly, there is no comprehensive study of emulsion spreading to date. In this research, the spreading of water/silicone oil emulsions on glass substrates was investigated. The time dependent variation of dynamic contact angle, base diameter, and the spreading rate of the emulsion droplets were studied. The effect of water/silicone oil weight percentage as well as the droplet size and dispersed phase bubble size were also investigated. The weight percentage of water/silicone oil emulsion and droplet size did not have a significant impact on the spreading dynamics; however the dispersed phase bubble size affected the spreading dynamics substantially. The coarsening of the dispersed phase bubbles was the key factor in the distinct spreading behavior of emulsions compared to pure liquids.

The only disadvantage of the Tensiometer is the fact of neglecting the viscous force in force measurement method applied on the plate’s surface during advancing and receding motion in the pool of highly viscous liquid. This neglect makes the Tensiometer to loose its extent of flexibility for being used for experiments with highly viscous liquids and for large liquid contact line speeds. To address this challenge, a viscous model for dynamic contact angle measurement using force-balance method with Tensiometer has been proposed.

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