Directing Colloidal Self-assembly Using Surface Microstructures

4 Directing Colloidal Self-assembly Using Surface Microstructures 

As indicated in Sect. 1.4.3, the depletion interaction depends on the overlap volume for a given depletant concentration. This dependence leads to a difference in depletion interaction between particles of different size and shape and offers a powerful and cost-effective way to separate them. 

The use of surface microstructures provides a promising route for creating colloidal assemblies via depletion forces. Dinsmore, Yodh and Pine [283] studied the interaction of large polystyrene spheres R = 203 nm, j) = 10 ) in a sea of small polystyrene spheres R = 41 nm, j) = 0.30) with a wall with a step edge, see Fig. 1.30. 

Clearly the overlap volume depends on the position of the big sphere with respect to the step edge. Since (1.21) 

This indicates that surface structures can create localized force fields which can trap particles. An interesting application of this concept can be found in the recent work of Sacanna et al. [284]. They created, by clever colloid synthesis, 5 pm (diameter) polymerized silicon oil droplets with a well-defined spherical cavity. To these lock particles they added appropriately sized spherical key particles (silica, poly(methyl methacrylate) or polystyrene coUoids) that can fit into the cavity. Nanometer sized non-adsorbing polymers were added to provide a depletion interaction. The depletion interaction, being proportional to the overlap volume of the depletion zones, attains a maximum when the key particle fits precisely into the spherical cavity of a lock particle, see Fig. 1.31. The depletion-driven self-assembly of lock and key particles is demonstrated in Fig. 1.32. This time series (from left to right) illustrates the site-specificity of the attraction. 

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