Effect of the colloid softness and concentration on droplet breakup and formation in microchannels*

As a sustainable way to replace surfactants, colloids can be used to control formation of dispersed drops.

In this work, we investigate the formation of colloid-stabilised drops in a microchannel. Dodecane is used as outer phase, while water containing polystyrene particles (PS) or pNIPAM microgels (MG) is used as drop phase to study the effect of particle softness and concentration. Using high-speed imaging, the dynamics of the drop formation were capture for both particle types.

Following classic film breakup analysis, two trends were identified during the colloid-free drop formation: starting with an inertia-controlled neck thinning, it is the well-known competition between capillary and viscous forces that causes the neck breakup. The transition to the capillary- viscous regime happens when the neck reduces by approximately 30% of its maximum size. In the presence of MGs, the characteristic time to reach the capillary-viscous regime decreases independently of the concentration while PS particles shift gradually this transition with increasing concentration. However, in both cases the colloids affect significantly the initial inertia regime. Finally, just after breakup, higher deformations of colloid-laden drops were observed especially for high concentration of MGs which can imply a new elastic force component.