Drying suspensions often leave behind complex patterns of particulates, as might be seen in the coffee stains on a table. Here, we consider the dynamics of periodic band or uniform solid film formation on a vertical plate suspended partially in a drying colloidal solution. Direct observations allow us to visualize the dynamics of band and film deposition, where both are made of multiple layers of close packed particles. We further see that there is a transition between banding and filming when the colloidal concentration is varied. A minimal theory of the liquid meniscus motion along the plate reveals the dynamics of the banding and its transition to the filming as a function of the ratio of deposition and evaporation rates. We also provide a complementary multiphase model of colloids dissolved in the liquid, which couples the inhomogeneous evaporation at the evolving meniscus to the fluid and particulate flows and the transition from a dilute suspension to a porous plug. This allows us to determine the concentration dependence of the bandwidth and the deposition rate. Together, our findings allow for the control of drying-induced patterning as a function of the colloidal concentration and evaporationrate.
Nanoporous architectures use periodic arrays of hollow spaces to generate the intense structural colors of butterflies, beetles, and birds, enhance the mechanical stability of bones, and facilitate gas exchange through egg shells. Analogous synthetic periodic nanoporous structures, known as inverse opals, offer a compelling materials strategy for use in optics as well as in fields ranging from catalysis and energy storage to tissue engineering. While inverse opals and other 3D photonic structures can be produced by top-down processes, a much simpler, lower cost approach to generating uniform pore size and order is to use self-assembling colloidal spheres to construct a patterned, periodic colloidal crystal, or opal, which then acts as a sacrificial template for self-assembling the porous structure. However, this technique has been plagued by uncontrolled crack and defect formation over the length scales required for most applications. We have discovered that taking a simpler approach - letting colloids and a silicate sol-gel precursor co-assemble in one step rather than sequentially – generates highly ordered, crack-free, multilayered inverse opal films on the scale of centimeters.
We are currently investigating the mechanism behind this long-range order; co-assembly not only avoids the cracking and inhomogeneities associated with liquid infiltration into a preassembled opal but also appears to take advantage of an interplay between the assembling template and matrix that leads to correction of incipient defects. The versatility of this approach enables us to fabricate hierarchical structures not achievable by conventional methods, such as introducing condensation-induced anisotropy.