Evolution of the Shape and Surrounding Stress-Strain of an Oil Droplet Embedded in a Drying Colloidal Suspension

During drying, colloidal dispersions undergo solidification where particles pack in the drying front and rearrange themselves to release stress. An immiscible liquid droplet embedded in the packed colloids was thought to be a potentially useful probe to map the stress distribution. To understand how the droplet deform in compacting colloids, we designed experiments that used time-lapse confocal microscopy to measure the movement and deformation of an oil droplet when colloids transited from a uniform suspension to compaction during uniaxial drying in a microfluidic channel. Fluorescent tracer particles in the colloids were used to map the displacement fields from which the stress-strain fields were obtained in the areas surrounding the oil droplet. We observed that an initially spherical oil droplet slowed relative to the surrounding fluid, suggesting diffusiophoresis of the droplet in the concentration gradient of the colloidal particles. As time progressed, the concentration of the colloidal particles grew, and the droplet was deformed by the hydrostatic pressure gradient. The distortion of the oil droplet showed that the elastic modulus of the nearby colloidal particles increased and overcame the surface tension of the oil droplet. At long times, when the colloidal particles were near closed packing, the shape of the oil droplet lost front-back symmetry – with higher curvature in the more packed front. The curvature of the droplet was found to be due to the hydrostatic pressure gradient rather than the colloidal osmotic pressure gradient which was in the direction opposite to that of the hydrostatic pressure gradient. Video images, analysis of the droplet slowing down due to diffusiophoresis, time evolution of the shape of an oil droplet and the stress-strain field surrounding droplet will be presented at the talk.