(315d) Microfluidic Generation of Pickering Drops With Independently Controlled Drop Volume and Particle Surface Concentration

Particle stabilized emulsions, known as Pickering Emulsions, are found in food, energy applications, cosmetics, and pharmaceuticals. Pickering emulsions’ stability occurs due to mechanical protection provided by and the enhanced interfacial rheology of individual drop’s particle laden interface.  Interfacial rheology is a strong function of particle surface concentration; it also affects drop deformation, which impacts bulk emulsion rheology.  However, due to the difficulty in creating Pickering drops with controlled surface concentration, the relationship between surface concentration, interfacial rheology and droplet deformation has not been explicitly characterized. 

We present a new microfluidic method to create Pickering drops with controlled particle surface concentration, which are then used to examine the role of surface concentration on deformation.  A microfluidic co-flow geometry is used to generate drops; the design is modified to introduce an inner dispersed phase flow composed of a particle dispersion sheathing flow which surrounds an inner aqueous flow. Due to the narrow width of the sheathing flow, particles are trapped in a region close to the interface during breakup, which ensures fast diffusion of all particles to the interface.   The Pickering drop surface concentration is a function of droplet volume and particle number within a drop.  Droplet volume is controlled by the Capillary number defined by an outer oil phase flow rate, and a Weber number defined by the total dispersed phase flow rate.  Particle number is controlled by bulk particle concentration in the sheathing flow and the flow rate ratio between the sheathing flow and the inner aqueous phase.  By adjusting Capillary number, Weber number, and inner phase flow rate ratio, we can independently control both particle surface concentration and drop volume in real-time.  By using mass conservation, particle surface concentration can be predicted; it is measured experimentally by polymerizing Pickering drops and imaging them using confocal microscopy. Experimental results are compared to predicted surface concentration. The role of particle surface concentration on droplet breakup regimes is also examined. Using these drops, we present initial results on the effect of particle concentration on droplet deformation in uniaxial extension; microfluidics are particularly advantageous to this work due to their ability to allow tracking of multiple length and time scales associated with the particles, their organization, surface flow, and drop deformation. 

We present a systematic study the generation of Pickering drops of controlled surface concentration and examine the effect of particle laden interfaces on generation and the deformation of these drops.