We describe a method to produce millimeter-sized hydrogel particles, by dispersing aqueous droplets in an oil using a nozzle and subsequently solidifying them. We show that we can vary the size of the particles using an air flow along the nozzle. The resulting particle size can be well predicted by a simple model where a drag force generated by the air flow, adds to the weight pulling the droplet from the nozzle. Particles produced using this method have diameters ranging from 0.7 to 2.3 mm. Production rates up to 0.5 ml/min per nozzle have been achieved, which compares favorably to standard microfluidic techniques. Finally, we show that the method can be used to produce both physical and chemical gel particles and is thus highly universal.

Abstract. We develop a method to investigate the microscopic origin of granular fluidity. We design a Couette cell in which we can probe the flow of soft hydrogel suspensions. As we drive the suspension with a rheometer, we have access to global flow characteristics. In addition, the Couette cell has been modified to have a transparent bottom and lid, allowing for imaging of suspension characteristics in transmission, for example flow fields. We can also use transmission imaging to probe local stresses in the suspension: we use hydrogel particles composed of gelatin, which through its photoelastic properties gives access to local stress fluctuations. We thus have access to all local microscopic variables that are relevant in the understanding of granular suspensions. We show here that our setup can indeed visualize stress fields inside the suspensions and perform flow field measurements in transmission mode. We compare the profiles of the gelatin suspension to that of a polyacrylamide hydrogel suspension, to assess robustness of observed phenomenology. We find that the flow profiles for both types of hydrogels are different; gelatin suspensions feature narrower and less rate-dependent flow profiles. We speculate on the origin of the observed difference by considering the frictional properties of the suspended particles.