Flow-induced fracture in freestanding wet colloidal pillars

Self-assembly of colloidal particles is an attractive means to create materials with engineered properties by controlling the hierarchy of particle composition, size, ordering and macroscopic form. We recently demonstrated a direct-write method [1] that enables self-assembly of colloidal particles from liquid suspension into centimeter-scale freestanding crystalline structures by utilizing a combination of evaporation-driven liquid flow and capillary attraction. However, in a subset of the structures we constructed, cracks formed during drying, throughout the height and at the free end. We detail our observations and explain how the Darcy flow of liquid through the structure drives fracture, despite the structure being compressed by a uniform capillary pressure at its surface. We derive a criterion for the presence and spacing between cracks, based on an energy scaling argument, that compares favorably with our experiments. This work ultimately provides a guideline for constructing crack-free 3D colloidal structures, which can potentially achieve complex freeform macroscale geometries.
[1] Tan*, Beroz*, Kolle, Hart, Adv. Mater, (2018).