Experimental study of colloidal particles confined in a 2D microfluidic droplet*

Particles are sometimes added during droplet formation to replace surfactants as surface active agents and improve the stability of the dispersions [1,2]. Generally, it is assumed that these particles follow classic Brownian motion. However, if the particles are confined their behaviour can be modified [3], as observed numerically [4] for spherical and cylindrical particles inside a rigid spherical cavity. However, the dynamics of confined particles in droplets formed in microfluidic channels is still poorly understood.

In the present work, the displacement of particles inside a fixed aqueous droplet in oil was investigated using fluorescently tagged polystyrene particles illuminated by a continuous laser. With this set up it is possible to track both time and spatial evolution of the particles inside the droplet at different particle concentrations. Quasi 2D-droplets were generated in a flat glass microchannel and the images were taken with a high-resolution camera equipped with a microscope lens at a frequency of 4Hz. The trajectories of the particles were obtained to calculate both radial and tangential mean square displacement (MSD). The results showed that the MSD behaviour changes significantly with the particles concentration and the radial location in the drop.