Dynamic Wetting by Viscous Liquids: Effects of Softness, Wettability and Curvature of the Substrate and Influence of External Electric Fields.
[Ph.D. Thesis], (2013)
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|Item Type:||Ph.D. Thesis|
|Title:||Dynamic Wetting by Viscous Liquids: Effects of Softness, Wettability and Curvature of the Substrate and Influence of External Electric Fields|
The wetting of solid surfaces by liquids is commonly observed in nature, and it is also a key to a number of industrial applications and biological processes. In the past two centuries, most studies about wetting were devoted mainly to equilibrium situations and thus to static measurements. However, in most cases the dynamic wetting is more relevant and it has received less attension. The goal of this thesis is to study the effects of softness, wettability and curvature of the substrate and influence of external electric fields on dynamic wetting of viscous liquids. The thesis contains two main parts. The first part focuses on the early dynamic wetting of simple liquids on two types of surfaces that show different complexity: flat viscoelastic substrates and highly curved solid microparticles. On the viscoelastic substrates, a novel wetting stage dominated by inertia was found. The dynamics in this stage is characterized by the wetting radius, r=K't^α, following a power law similarly as on rigid surfaces, with the exponent α only depending on surface wettability. After the inertial wetting stage, spreading slows down and enters another stage dominated by the viscoelasticity of the substrate. The transition between inertial and viscoelastic stage is controlled by the surface “softness”. A simple theory was developed with Prof. Martin E.R. Shanahan to explain these findings. An early inertial wetting stage was also observed during the snap-in process, i.e. the wetting, of single colloidal particles into large water drops. The snap-in time is dependent on the capillary force and on inertia, but is independent on surface wettability. In contrast, the snap-in force is larger for hydrophilic and smaller for hydrophobic particles. A scaling model was proposed to describe the snap-in or early wetting of individual colloids. The second part of the thesis is devoted to study the dynamic wetting of rigid flat surfaces by simple and viscous liquids. First, the early spreading of drops of aqueous electrolyte solutions on various wettable surfaces driven by electrostatic forces, which was termed “electrospreading”, was investigated. It was found that early electrospreading is only dominated by inertia and electrostatics. The wetting dynamics is not only dependent on surface wettability and applied electric potential, but also on the concentration of the electrolyte solutions. The electrostatic energy stored in the electric double layer near the solid-liquid interface served as an additional energy for driving drop spreading. Based on molecular dynamics simulation done by Dr. Chunli Li, a simple scaling model was presented to describe the wetting dynamics. Finally, a systematic study of dynamic wetting of various wettable surfaces by viscous liquids was carried out. Both surface wettability and liquid viscosity influence the inertial stage of wetting as well as the viscous stage. During the inertial wetting stage, the effective mass of the spreading drop is affected by surface wettability and liquid viscosity. This results in a slower spreading speed on hydrophobic surfaces, or of highly viscous liquids. Viscous wetting did not take place on all substrates, but only on those surfaces with equilibrium contact angles smaller than a critical value, which depended again on liquid viscosity. A scaling law was proposed to explain these experimental observations.
|Place of Publication:||Darmstadt|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 530 Physik
600 Technik, Medizin, angewandte Wissenschaften > 620 Ingenieurwissenschaften
|Divisions:||Exzellenzinitiative > Clusters of Excellence > Center of Smart Interfaces (CSI)|
|Date Deposited:||12 Dec 2013 11:35|
|Last Modified:||20 Jul 2016 12:49|
|Referees:||Tropea , Prof. Cameron and Bonaccurso , PD Elmar and Butt , Prof. Hans-Jürgen|
|Refereed:||1 December 2013|