Droplet evaporation on smooth patterns

Ewetola, Michael (2021). Droplet evaporation on smooth patterns. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0001343b

Abstract

Controlling the shape and location of evaporating droplets on a solid surface is critical in a variety of industrial applications where droplet dynamics is important. Ink-jet printing, display technologies, DNA analysis, lab-on-a-chip device design, coating, micropatterning, and heat transfer are examples. This thesis investigates how to control droplet evaporation on smooth surfaces with well-defined patterns.

The evaporation of a two-dimensional droplet on a solid surface is initially investigated. The solid is flat, but there are smooth chemical variations that cause a space-dependent local contact angle. We conduct a detailed bifurcation analysis of the droplet’s equilibrium properties as its size changes, observing the emergence of a hierarchy of bifurcations that is strongly dependent on the underlying chemical pattern. Symmetric and periodic patterns give rise to a series of pitchfork and saddle-node bifurcations, causing stable solutions to become saddle nodes. This change in stability under dynamic conditions implies that any perturbation in the system can cause the droplet to shift laterally while relaxing to the nearest stable point in a snap event, as confirmed by numerical computations of the Cahn-Hilliard and Navier-Stokes systems of equations. Also, with asymmetric patterns we are able to effectively control droplet evaporation.

This enables the investigation of the effect of wetting strength and droplet properties such as size, viscosity, and surface tension on the snap speed of evaporating droplets on a smooth pattern. We investigate the interaction of chemical and topographical patterns on non-planar surfaces. We discover that combining periodic chemical and topographical patterns can either amplify or annihilate snap effects. A well-defined wetting pattern can also be used to control the direction of evaporating droplets on a non-planar surface.

Gravity’s effect on evaporating droplets on a flat surface with periodic wetting patterns is also investigated. Gravity changes the shape of a droplet by increasing its radius and decreasing its height while keeping its size constant. As it evaporates on an incline plane with a symmetric chemical pattern, a droplet will prefer to move in the direction of gravity. However, it is discovered that when the effect of gravity is weaker than the strength of the wetting pattern, a droplet can be made to move in the opposite direction dictated by gravity on an asymmetric pattern with amplitude gradient.

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