Experimental investigation on the dynamic wetting of deformable substrates: Influence of the rheology of substrate and wetting liquid and external body force

Abstract

Wetting of surfaces by liquids is observed in many natural and technical processes. Efficient wetting of surfaces can lead to technological advancement in the field of aeronautics, medical and forensic science, cosmetics etc. In all these applications complex liquids wet complex surfaces. Wetting investigation of textured and soft surfaces has gained popularity due to its relative importance in the aforementioned fields. Soft surfaces provide a unique feature through their capacity to deform when subjected to external forces. The unbalanced surface tension force arising from the normal component of the surface tension force, results in pulling the surface upwards at the triple phase contact line. Combining this deformation at the contact line, with the deformation due to the Laplace pressure at the contact area results in a crater like shape. This deformation at the contact area shortens evaporation time, influences particle deposition pattern and drop spreading kinetics. The effects arising from the elasticity and the surface tension of the soft substrates have been combined together what is being known as Elastocapillary effects. The knowledge of the effect not only helped in getting a new perspective on the behavior of soft substrates but also gave an explanation to the many observed phenomena that were previously unexplained. The goal of this thesis is to study the effects of softness, viscoelasticity and external body force on the dynamic wetting of soft substrate and non-Newtonian liquids. \nThe experimental investigations in this thesis span over two main areas. Firstly, the influence of the substrate and liquid viscoelasticity on initial stages of spreading and drop impact outcomes is investigated. Secondly, the influence of the external body force on the spreading and sliding of viscous drops on soft surfaces is studied. Performed spreading experiments with Newtonian and non-Newtonian liquids showed that the early drop dynamics can be described by a power law r = Ct^n. However, on hard surfaces the power law exists during the entire duration of the spreading process whereas on soft surfaces there is a transition to a slower spreading rate which is dominated by the viscoelastic dissipation at the contact line. The transition time is dependent on the softness of the substrate. Spontaneous wetting experiments with non-Newtonian drops showed faster contact line velocity as compared to the Newtonian drops. In the case of drop impact, several new outcomes were observed for non-Newtonian liquids impacting hydrophobic soft surfaces. Fingering-splashing was observed for non-Newtonian drop splashing on soft surfaces. This phenomenon was prominent for polymer solutions with high polymer molecular weight. A regime map related the outcome of the experiments with the viscoelasticity of the substrate. \nInvestigation on the influence of the external body force on the spreading and sliding of drops of Newtonian liquids with different viscosities rest on soft surfaces was carried out. A custom built device called Kerberos (Division of Chemical Technology, Aristotle University of Thessaloniki) facilitated the experiments. A rotating arm helped to impart the rotational force on the drop. The drop underwent increasing tangential force while maintaining the normal force components zero. The drop spread under the tangential force and started to slide after a critical rotational speed was reached. The monitoring of the drop during the spreading and sliding revealed that on hard surfaces the drop spreads and slides keeping its semi-spherical shape intact. On soft surfaces the drop was severely deformed. It was also observed that the viscous drops started to spread and slide at a lower rotational speed than low viscosity liquids. A relationship between the equilibrium shear modulus of the substrate and the aspect ratio of the deformed drop was established.