A Droplet-Based Microfluidics Route to Temperature-Responsive Colloidal Molecules

Small clusters of spherical colloids that mimic real molecules, so-called colloidal molecules, hold great promise as building blocks in bottom-up routes to new materials. However, their typical hard sphere nature has hampered their assembly into ordered structures, largely due to a lack of control in the interparticle interactions. To provide easy external control of the interactions, the present work focuses on the preparation of colloidal molecules from temperature-responsive microgel particles that undergo a transition from a soft repulsive to a short-range attractive state as their characteristic volume phase transition temperature (VPTT) is crossed. Preparation of the colloidal molecules starts with the use of a droplet-based microfluidics device to form highly uniform water-in-oil (W/O) emulsion droplets containing, on average and with a narrow distribution, four microgels per droplet. Evaporation of the water then leads to the formation of colloidal molecule-like clusters, which can be harvested following cross-linking and phase transfer. We use a mixture of two types of microgels, one based on poly(N-isopropylacrylamide) (PNIPAM) and the other on poly(N-isopropylmethacrylamide) (PNIPMAM), to prepare bicomponent colloidal molecules, and show that the difference in VPTT between the two allows for induction of attractive interparticle interactions between the PNIPAM interaction sites at temperatures in between the two VPTTs, analogous to the interactions among patchy biomacromolecules such as many proteins.