The study and manipulation of liquid interfaces

Liquid interfaces are prevalent in all walks of life, and the study of surface science is widely applicable in fields ranging from microfluidics to semiconductor fabrication. In particular, the ability to manipulate liquid has tremendous potential in the field of microfluidics. It is widely sought after in analytical research and medical diagnostics. In order to develop new methods of manipulating small volumes of liquid, we must first understand the mechanics and behaviour present at the microscopic scale. The aim of this thesis is to improve our understanding regarding the interaction and behaviour of the liquid-solid and liquid-air interfaces of drops and bubbles across multiple length scales. The research can be divided into three interconnected categories: the interaction between microbubbles and the contact line of a liquid drop; the interaction between a liquid drop and a bubble of comparable size; and the characteristics of a bubble on a superhydrophobic surface. In all these, bubbles ranging in size from nanometres to centimetres have a chance to interact with other liquid interfaces. The presence of microbubbles within a drop was observed to cause disruptions along the contact line. In contrast to static features such as patterned surface structure or chemistry, entrained microbubbles naturally vary in size over time. These microbubbles may therefore be responsible for a newly identified time-dependent retention force, i.e., the force that must be overcome for a drop to move. The results presented in this thesis fill a gap in knowledge regarding drop mechanics, and has implications in the study of drop control for applications in microfluidics. For bubbles that are comparable with the size of the drop (several orders of magnitude larger than microbubbles), a single encapsulated bubble was capable of interacting strongly with the liquid-solid and liquid-air interfaces of the encapsulating drop. This enabled pinning of the drop beyond the typical sliding incline. The rupture of the encapsulated bubble produced capillary waves which travelled across the liquid-air interface of the drop. On a horizontal superhydrophobic surface, the released surface energy propelled the entire drop vertically off the substrate. For drops pinned on an incline, bubble rupture resulted in controlled drop detachment. The combined information provides us with a new and simple method of controlling the motion of drops. While large bubbles cannot be introduced into a drop on a superhydrophobic surface due to air dissipation, this can be overcome by immersing the surface within a liquid pool, therefore enabling the study of pendant bubbles. In studying their characteristics, it was found that by interacting with micro- and nanobubbles trapped on the substrate, the pendant bubbles were able to assume sizes larger than those predicted theoretically and numerically. Close examination also found them to be superior to the conventional sessile drop method for the contact angle characterization of superhydrophobic surfaces. The improved accuracy is equivalent to an increase of 2 orders of magnitude in the pixel count of a sessile drop imaging system. The results presented in this thesis advances our knowledge regarding the interaction and behaviour of liquid interfaces. Along with the potential applications identified here, it is hoped that the results will allow us to further explore new methods of both understanding and controlling liquids in the future.