Modulation of Spatio-temporal Particle Patterning in Evaporating Droplets: Applications to Diagnostics and Materials Science.

Spatio-temporal particle patterning in evaporating droplets lacks a common design framework. Here, we demonstrate autonomous control of particle distribution in evaporating droplets through the imposition of a salt-induced self-generated electric field as a generalized patterning strategy. Through modeling, a new dimensionless number, termed "capillary-phoresis number" (CP), arises which determines the relative contributions of electrokinetic and convective transport to pattern formation, enabling one to accurately predict the mode of particle assembly by controlling the spontaneous electric field and surface potentials. Modulation of CP allows the particles to be focused in a specific region in space or distributed evenly. Moreover, starting with a mixture of two different particle types, their relative placement in the ensuing pattern can be controlled, allowing co-assemblies of multiple, distinct particle populations. By this approach, hypermethylated DNA, prevalent in cancerous cells, can be qualitatively distinguished from normal DNA of comparable molecular weights. In other examples, we show uniform dispersion of several particle types (polymeric colloids, multi-walled carbon nanotubes and molecular dyes) on different substrates (metallic Cu, metal oxide and flexible polymer) as dictated by CP. Depending on the particle, the highly uniform distribution leads to surfaces with lower sheet resistance, as well as superior dye-printed displays.