Controlling three-dimensional ice template via two-dimensional surface wetting

Abstract

Directional freezing (DF) is a fast, scalable, and environmental friendly technique for fabricating monoliths with long-range oriented pores, which can be applied toward a wide variety of materials. However, the pore size is typically larger than 20 μm and cannot be spatially controlled, which prevent the technique from being used more widely. In this work, effect of wettability of the freezing substrate on the pore size of monolithic polyethylene glycol cryogels is studied. Smaller pores can be generated via more hydrophilic substrates, and tubular pores smaller 5 μm can be created using a poly(vinyl alcohol) coated copper substrate. A numerical fitting between water contact angle of the substrates and pore size is then obtained. Moreover, pore size can be locally varied duplicating wetting patterns of the substrates. The concept of using two dimensional patterns to build monoliths with three dimensional microstructures can probably be extended to other material systems. DF is an effecient and scalable processing method for fabricating materials with long-range oriented pores. However, the smallest pore feature size reported is around 20 µm, which is in many cases too large for application such as separation and catalysis. We show here, with exemplary cryogels, that both spatial control and feature size reduction (by one order of magnitude) can be realized in DF by controlling the wettability of the ice growth substrate.