Stress-Strain relaxation in regions surrounding an oil droplet in a Drying Colloidal Suspension

Colloidal dispersions of submicron particles pack and solidify upon drying, however, how stresses are relaxed in regions surrounding embedded oil droplets in an aqueous suspension of colloidal nanoparticles is unknown. This study uses time-lapse confocal fluorescence microscopy to measure drying-induced movements of micron-sized fluorescent tracer particles and deformation of oil droplets in a linear microfluidic channel. Tracer particles were used to map the displacement fields from which the stress-strain fields of the nanoparticles surrounding the oil droplet. The shape of the oil droplets, initially spherical, deform into oblate spheroidal in the concentration gradient of the colloidal particles surrounding them. At long times, when the colloidal particles approach close-packing, the oil oblates lost front-back symmetry – with higher curvature in the more packed front – suggesting an intriguing interpretation that the pressure in the colloidal particle domain is lower at the oil-water interface in the front side facing downstream than the upstream side. The time-dependent stress-strain fields are used to analyze the stress relaxation by considering the force balance between the hydrostatic pressure, colloidal osmotic pressure, interfacial tension of the oil droplets. Comparison of experimental data with diffusion equation-based numerical simulation provides an additional angle to understand the stress relaxation of colloidal nanoparticles surrounding deformable obstacles.