Weakly interacting colloidal particles, with uniform sizes ranging from several nanometers to microns, can spontaneously organize into close-packed crystals from concentrated liquid suspensions. Because they provide a simple, ordered structure with well-controlled and homogeneous porosity, these materials have been studied for many important applications, including sensing, separations, microfiltration, and batteries. A particularly interesting and promising application for colloidal crystals is their role for fabricating photonic crystals. These crystals exhibit a band gap for photons; namely, there exists a range of photon frequencies inside the material for which light cannot propagate in any direction. This property could be utilized to manipulate photons for novel optical circuits, biological and chemical sensors, and efficient thermal emission sources. To advance all of these applications, there is a need for an efficient, low-cost means to manufacture large quantities of high-quality colloidal crystals.

Colloidal crystals have traditionally been made via the gentle sedimentation of spheres in a liquid suspension. This technique is ill-suited as a growth process, since the settling rate is very slow, requiring months. If rushed, the resultant crystal is typically flawed by a significant amount of disorder. A process known as convective self-assembly can quickly, within hours, deposit colloidal particles into layers onto an inclined plate immersed within an evaporating liquid suspension. Beyond all thermodynamic reasoning, these vigorously growing layers are characterized by a nearly perfect, face-centered cubic (fcc) crystalline structure, the equilibrium packing for this system. The fast growth rate and high material quality make convective assembly an attractive candidate for a manufacturing process for colloidal crystals.

In this presentation, we discuss the role of solvent flow in guiding assembly of colloidal crystals. One idea is that solvent flow through the pore space of close-packed spheres could direct advancing spheres into the clear niches, where flow is higher than into obstructed niches. The action of flow into and across the crystal is argued to reinforce the tendency of a new layer to grow with the desired fcc packing. We present the results of theoretical and experimental studies to test this hypothesis and to better understand the role of fluid flow during convective assembly.