Direct force measurement of microscopic droplets pulled along soft surfaces

Recently, there has been growing interest in understanding the interactions of liquid droplets with soft materials. In these systems, forces exerted by a droplet can deform the material it contacts. This property leads to a plethora of unique physical phenomena with applications in fields ranging from water collection to surface sensing. We explore droplet dynamics on soft materials using a micropipette-based technique to simultaneously image, and measure the forces on, a microscopic droplet dragged along a soft interface. By changing the thickness of the soft material, we can control the compliance independent of surface chemistry. We show that material stiffness is a key parameter in droplet dynamics.