Three-Dimensional Deposits from Drying Particle-Laden Drops

Formation of inhomogeneous (in the form of a "coffee ring") or homogeneous deposits accompanies the drying of a particle-laden drop. Invariably, this deposition occurs in a two-dimensional (2D) space (x, y plane) (and might have a finite thickness in z), where the evaporating drop is positioned. Here, we show an interesting extension of this problem: we demonstrate the occurrence of evaporation-mediated particle deposits that span three dimensions (x, y, and z). The extent of the span in this 3rd dimension (z) is comparable to the span in x and y and hence is much larger than the finite thickness (in z) of the 2D deposits. Particle-laden drops are introduced in an uncured and heavier (than the drop) polydimethysiloxane (PDMS) film, enabling the drop to come to the uncured PDMS surface and breach it and get partly exposed to the surrounding air enforcing the onset of evaporation. The subsequent curing of the drop-laden PDMS film ensures that the drop is occupying a three-dimensional (3D) cavity; as a consequence, the evaporation-driven flow field, depending on the particle sizes, leads to a deposition pattern that spans three dimensions. We consider particles of three different sizes: coffee particles (20-50 μm), silver nanoparticles (∼20 nm), and carbon nanotubes (CNTs) (1-2 μm). The coffee particles form a ring-like deposit in the x, y plane, while the much smaller silver nanoparticles (NPs) and CNTs form a 3D deposit that spans in x, y, and z directions. We anticipate that the present finding of the evaporation-triggered three-dimensional (3D) particle deposits will enable unprecedented self-assembly-driven fabrication of various materials, structures, and functional devices as well as patterning and coating in 3D spaces.