Highly mobile droplets on slippery surfaces

Droplets on slippery surfaces such as Slippery Liquid Infused Porous Surfaces or Liquid - Impregnated Surfaces (SLIPS/LIS) are inherently highly mobile and therefore can be difficult to control in terms of position and motion. This thesis has explored how to return droplet control back to very slippery surfaces via four interconnected studies. The first study investigated how to return droplet control back to these slippery surfaces by adding a simple step structure to the surface. The adhesive force created by this structure has been quantified by measuring the droplet sliding angle as a function of step height and oil thickness. As the step height was increased so did the adhesive force, as the oil thickness was increased the adhesive force decreased. Given the correct conditions the adhesive force can be great enough to hold a droplet upside down. The ability of the structure to produce motion via a repulsive or attractive force has been investigated in the second study by changing where the droplet was placed onto the substrate. The type of force produced was found to not only vary with position but with oil thickness and time. The third study involved a more complex topography onto which the SLIPS coating was applied, creating a mobile surface which also shaped the droplet. As a droplet placed on to this surface evaporated it was shown that the shape of the droplet could be accurately predicted for a given droplet volume. The final study developed a SLIPS coating that could be applied to any structure, even a closed geometry, in one single step, producing sliding angles equivalent to those seen on a standard SLIPS coating. This coating has also been shown to be long lasting with the SLIPS properties remaining more than two years after it was first produced and tested.