Shape-Selectivity in the Aggregation of Lithographically-Designed Colloidal Particles

Photolithography can be used to form monodisperse colloids of well-defined, non-spherical shape in a negative photoresist, SU-8. In aqueous suspension, in the presence of dextran as a depletant, we showed that the aggregation of these particles was highly selective for the end-to-end configuration [1]: the cylinders assembled into linear aggregates that could extend to lengths of tens of units without significant lateral aggregation. We have performed an in-depth study of the mechanisms by which these particles aggregate. We focus on the roles of global shape, roughness, and adsorbed layers of surfactants in mediating depletion, van der Waals (vdW), and electrostatic interactions between these particles [2]. Summary of Research: Methods: As illustrated in Figure 1A, we generate colloidal particles by direct lithography. The key aspects of the process are: 1) Spin coating of a sacrificial layer (Omnicoat, Microchem corp.) on a 100 mm silicon wafer; this layer is later dissolved in order to release the particles. 2) Spin coating of a layer of SU-8; this step defines the height, h, of the particles that will be formed. 3) Exposure of the SU-8 layer to ultraviolet light (GCA Autostep 200 DSW i-line Wafer Stepper) through a photomask; this step defines the diameter, d, of the particles. 4) Development of the resist, release of the particles via dissolution of the sacrificial layer, washing via repeated centrifugation, and resuspension in a buffered aqueous solution of Tergitol NP40, a nonionic surfactant. This route to the formation of particles generates ~ 109 colloidal particles on a 100 mm wafer. We have formed particles with thicknesses in the range 0.3 < H < 3 μm, and diameters in the range 0.4 < Rp < 4 μm. We characterized surface roughness of particles via atomic force microscopy, zeta potential via dynamic light scattering, Hammaker constant via spectrally resolved ellipsometry, and structure of assemblies by optical microscopy. Figure 1: (A-B) Fabrication and release of particles. (B) Depiction of modes of interaction and roles of surface properties. (D) Diagram of steric stabilization layers.