Evaporation of sessile droplets on pinning-free surfaces

Armstrong, Steven (2020) Evaporation of sessile droplets on pinning-free surfaces. Doctoral thesis, Northumbria University.

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Abstract

Contact-line pinning is a fundamental limitation of diffusion-limited evaporation of sessile droplets. Sessile droplet evaporation is pervasive in a wide range of situations from ink-jet printing, to pesticide sprays and spotted microarrays. Contact-line pinning drives, for example, stick-slip motion and non-uniform deposition of solute within the droplet. Moreover, contact-line pinning is problematic in a wide range of situations, such as droplet microfluidics where capillary forces dominate the motion of liquid fronts.

Recently, Slippery Liquid-Infused Porous Surfaces (SLIPS) have shown excellent droplet shedding abilities by use of a lubricating liquid, imbibed into a porous structure, immiscible to droplets on the surface. However, the lubricating liquid removes the droplet-solid-interaction, can cloak the droplet, can be several microns in thickness and the porous structure can be fragile to external mechanical forces. Slippery Omniphobic Covalently Attached Liquid-Like (SOCAL) is a new type of liquid-like surface, which is an attached coating rather than a retained liquid. SOCAL promises pinning-free properties while being nanometres thick and demonstrating mechanical robustness. Few research groups have reported successful creation of SOCAL surfaces.

This thesis shows an optimised methodology to make reliably, pinning-free low-hysteresis SOCAL surfaces. This is done by modifying the parameters to create SOCAL and measuring the contact-angle hysteresis. A low contact-angle hysteresis of < 1° is achieved. These surfaces then show, for the first time, constant contact angle mode evaporation of sessile water droplets from a solid surface. This allows for the accurate measurement of the diffusion coefficient of water. An unexpected feature of the evaporation sequences is a step change increase in contact angle reminiscent of a type V adsorption isotherm. Attempts are made to characterise this using Dynamic Vapour Sorption (DVS) and Quartz Crystal Microbalance (QCM) techniques.

This thesis also shows voltage-programmable control of water droplets on SOCAL using electrowetting. The unexpected behaviour of droplets on SOCAL is investigated and the electrowetting device is optimised. This allows control of the constant contact angle evaporation on both SLIPS and SOCAL. This is used to study the effect on the contact angle during the evaporation of sessile water droplets. The results of this thesis will benefit the aforementioned applications overcoming contact-line pinning and introducing new methods of controlling sessile droplet evaporation.

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