Driven motion in droplets

Abstract

This work is a “sandwich” thesis, containing the work of 4 manuscripts studying droplet motion preceded by background chapters. We start with an introduction that focuses on general concepts in capillarity and fluid dynamics, and how we can build scaling models from first principles. Following the introduction, there is a methodology chapter which provides some notes on the experimental methods used in the manuscripts. In the first manuscript (chapter 3) we look into how a moving a droplet leads to dissipation in an underlying underlying soft substrate. We develop a system of sub-micron elastomeric substrates as well as micropipette based technique to study the forces on micrometric droplets in motion. We find that dissipation scales with the thickness of the underlying film. In chapter 4, we follow up the work on dissipation in soft substrates with a study on the role of uncrosslinked chains in the same substrates, as well as providing more details on substrate fabrication. Next, in chapter 5 we study how geometry can be used to drive motion in droplets. We suspend droplets between two fibers held at an angle and find droplets move towards the apex of the fibers. We develop a simple scaling model for the motion and we are able to modify the fibers to develop a droplet pump that allows for long range microdroplet transport. In the final manuscript (chapter 6) we study how external forces can be used to drive droplet motion. In particular, we study how magnetic fields can drive rearrangements in an aggregate of ferrofluid droplets. We describe phase changes in such a system with a simple scaling model. In these works we develop an understanding of how to drive motion in droplets, with an impact on both fundamental physics and applications in industry.