Role of interfacial rheology on the dynamics of confined droplets.

The development of droplet-based microfluidics addresses the question of predicting the velocity of these droplets when they are pushed by a carrier phase. In this work we focus on non-wetting droplets, i.e. there is a lubrication film between the droplet interface and the channel wall. These droplets are squeezed between the channel walls, i.e. they adopt a pancake-like shape. It is well known that the dynamics of droplets depend strongly on the physicochemical properties of the solutions, in particular on the solubility of the surfactant that is used.  Typically, an accumulation of surfactant at a stagnation point of the interface can generate Marangoni stresses, leading to an increased dissipation in the lubrication film, and thus to a decrease in droplet velocity. We report experimental studies on the dynamics of droplets in a microfluidic Hele-Shaw cavity, by investigating the impact of the solubility of the used surfactants on their velocity. By doing so, we highlight that the droplet velocity dependence on their radius is reversed as soon as the ionic surfactants used have more than 12 carbons on their hydrophobic tail.

In order to investigate the local signature of the surfactant along the lubrication film, we have set up a Reflection Interference Contrast Microscopy (RICM) device with which the thickness of the lubrication films can be measured. Able to reconstruct the topography of the droplet/external phase interface, the local properties of the interface can be extracted.