Droplets on Low Friction Surfaces

Guan, Jian (2017) Droplets on Low Friction Surfaces. Doctoral thesis, Northumbria University.

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Abstract

Droplet mobility on surfaces is often hampered by the pinning of the droplet’s contact line. External forces would be needed if motion is to continue. The development of Slippery Liquid-Infused Porous Surfaces (SLIPS) or Lubricant-Impregnated Surfaces (LIS) has since enabled the studies of droplets in low friction situations with virtually no contact angle hysteresis. This thesis presents three separate studies of droplet mobility in the absence of contact line pinning, made possible by the use of SLIPS/LIS. Firstly, the first study of evaporation of sessile droplet under true constant contact angle mode was demonstrated. The lack of contact line pinning meant that droplet’s contact line receded smoothly with no stick-slip stepwise retreat as it evaporated. The absence of a contact angle due to the presence of the wetting ridge around the droplet led to the concept of an extrapolated apparent contact angle. The subsequent study saw the experimental realisation of both inward and outward motion of droplet having an apparent contact angle above 90° confined in a wedge geometry formed by a pair of SLIPS/LIS. Out of equilibrium, droplet was free to travel within the wedge until settling at a prescribed location, regardless to its initial position. This enabled the accurate control over the transport and localisation of the droplet by a reconfiguration of the system. The lack of pinning also suggests that the dynamics of the droplet is dictated purely by viscous dissipation. The final study showed that SLIPS/LIS can be created on macro-patterned surfaces. The macro-patterning introduced menisci features in the impregnating liquid layer which interact with the wetting ridge around a droplet. These interactions were used to induce motion to the droplet. It was also found that accurate positioning of the droplet on an otherwise completely slippery surface can be achieved using well-defined surface topography.

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