(373a) Probing Colloidal Aggregation through Concentrating Processes in Microscale Droplet Reactors

Microscale droplet reactors offer a unique method of probing the stability of colloidal suspensions. Highly uniform picoliter volume droplets containing an aqueous suspension of nanoparticles are generated in a microfluidic device. The droplets are trapped hydrodynamically in the device, allowing the contents to be monitored over very long timescales.  Dehydration of the droplets results from slight solubility of the water in the outer oil phase, leading to increasing concentration of the encapsulated suspension.  Charge stabilized suspensions of silica nanoparticles containing little or no added salt are observed to dehydrate to a finite droplet size.  Regardless of the conditions driving the dehydration process, the final droplet reactor volume corresponds to a silica volume fraction of approximately Φ = 0.6, which is close to the maximum volume fraction for a random close packed suspension of hard spheres. When more salt is added, the droplet reactor stops shrinking at a significantly lower final particle volume fraction. A distinct visible structure forms inside these droplets at late times, and the entire droplet fractures and expels material from the inside of the drop.  We rationalize this behavior by considering the evolving composition of the droplet reactors. Salt concentration increases along with particle volume fraction during dehydration. For small initial salt concentrations, the final composition remains at or below the critical concentration expected for flocculation of charge stabilized silica suspensions. For larger initial salt concentrations, the final composition exceeds the critical flocculation concentration.  Probing the microrheology of the suspension within the droplets using fluorescent tracer particles reveals that, at fixed composition, the viscosity increases dramatically at timescales consistent with expected flocculation timescales for these suspensions.  The observed timescales are rapid enough at high added salt that flocculation is expected to occur rapidly at later stages of dehydration. These observations suggest that compressive stresses developing in a flocculated network may arrest droplet dehydration at lower volume fractions. The loose networks of particles may also permit salt crystals to form within the interstitial spaces. Thus, microfluidic droplet reactors provide a useful means of probing both fundamental stabilization mechanisms for colloidal suspensions, as well as the mechanics and structure of highly concentrated suspensions not easily accessible macroscopically.