Kraft Lab - Self-assembly in Biological and Soft Matter

Anisotropic Colloids

While colloidal aggregates are often undesirable due to their random shapes, we exploit them as a starting point to synthesize patchy particles.

Colloidal Particles with Tunable Surface Morphology

Surface morphology is a tool to tune physical properties of colloidal suspensions such as the wettability, viscoelasticity, and depletion attractions. We developed a synthetic approach based on swelling cross-linked latex spheres that can achieve spherical, dimpled and crumpled hybrid colloids with tunable surface roughness. These particles can furthermore be coated with silica, resulting in charge-stabilized hybrid particles and shells with controlled surface morphologies. 

Colloidal Recycling: Reconfiguration of Random Aggregates into Patchy Particles

We achieve a reconfiguration of the aggregates into uniform structures by swelling the polymer spheres with an apolar solvent. The swelling lowers the attractive van der Waals forces, lubricates the contact area between the spheres, and drives the reorganization through minimization of the interfacial energy of the swollen polymer network. This “colloidal recycling” approach yields patchy particles and clusters with well-controlled size distribution.

Synthesis of Organosilica Spheres

We are studying a chemical compound (trimethoxysilyl propyl methacrylate) that – given the right circumstances – spontaneously forms billions of droplets of exactly 0.0015 mm in diameter. As the material of these droplets is in between glass and plastic, many existing coatings can be applied to these colloidal spheres. This makes this material an easy building block for fabricating more complicated microstructures.

Self-Assembly of Colloidal Molecules with Liquid Protrusions

We exploit the coalescence of liquid protrusions on cross-linked latex particles to assemble colloidal molecules. Surface energy minimization leads to controlled shapes which can be further controlled by tuning the wetting angle and protrusion size. This enables the fabrication of colloidal molecules with tunable patchiness.

With this technique, we can also synthesize colloids with a well-controlled a bond angle, such as “colloidal water” and “colloidal ammonia”. This is achieved by increasing the degree of swelling to the point where the seed particles can freely move inside the fused liquid protrusions. Polymerization induces clustering, which leads to well controlled bond angles that can be "made-to-order" by adjusting the swelling ratio 

By tuning the synthesis conditions, we can control the number of protrusions appearing on the surface of highly cross-linked polymer spheres. High particle yields above 90% are achievable. We employed the technique for polyNIPAM, polystyrene, and magnetite filled polyNIPAM spheres. Partial roughness is achieved through adsorption of secondary particles, or wrinkling of the seed particle surface.