Colloidal suspensions have been of interest over the past decades as building blocks for advanced materials such as photonic crystal, chemical sensor, and drug delivery vehicle. Additionally, they are nice model systems for soft condensed matter to study phase behavior. Suspension of colloidal hydrogels, especially pNIPAm-co-AAc which swells/deswells in response to external stimuli (e.g. pH, ionic strength, temperature, and electromagnetic waves), have been extensively investigated. With temperature change, pNIPAm undergoes a volume phase transition around 30 degrees Celsius. Besides, by controlling co-monomer ratio (NIPAm:AAc), one can tune the electrostatic repulsion which, in turn, allows us to manipulate particle swelling effect upon pH change.

The phase behavior of hard spheres is well known and relatively simple. In contrast, the phase behavior of soft sphere is still poorly understood. In this work, we study the phase behavior of soft colloidal suspensions of pNIPAm-co-AAc hydrogels, whose volume can be controlled by pH and temperature, via particle tracking microscopy in a microfluidic device. The device provides us direct control over solvent composition. By varying pH, we can induce volume fraction changes, resulting in phase transition from liquid to crystal or vice versa. As a result, we are able to investigate phase behavior of hydrogels as a function of volume fractions in real time. While with gradual pH increases we probe equilibrium phase behavior, with sudden increases of pH we can investigate jamming, a kinetically trapped state. In contrast to hard spheres, soft hydrogel suspensions display a broad liquid/crystal coexistence domain with Random Hexagonal Close Packing crystal structure.