| We investigate evaporative lithography as a novel route for patterning colloidal films. Specifically, we dry colloidal suspensions beneath patterned masks that induce periodic variations between regions of free and hindered evaporation. Three types of colloidal films are studied: (i) aqueous, unary systems, (ii) non-aqueous, unary systems, and (iii) binary microsphere-nanoparticle systems. Through a combination of experiment and finite element modeling, we study the remarkable pattern formation induced by evaporative lithography, and establish the guidelines for controlling particle segregation in the drying films.;Direct imaging of aqueous colloidal films, dried beneath a patterned mask, reveals that particles segregate laterally within the film, as fluid and entrained particles migrate towards regions of higher evaporative flux. Pattern formation can be regulated by tuning the initial suspension composition, separation distance between the mask and underlying film, and mask geometry. We show that discrete patterned features are formed when the initial colloid volume fraction is low, and continuous patterned films form at higher initial volume fractions. A critical value of the ratio of the separation distance between the mask and underlying film and the center-to-center distance between unmasked features is identified, below which, particle segregation is observed. Above the critical value, variations in the evaporative flux profile are too small to drive significant particle segregation.;To investigate the evaporative lithographic patterning of multiple species, we prepare binary colloidal films composed of microsphere-nanoparticle mixtures. When the microsphere volume fraction greatly exceeds that of the nanoparticles, the microspheres consolidate to form a continuous patterned film, while the nanoparticles remain entrained in the fluid. Capillary forces arise when the drying front recedes into the microsphere network and wick the nanoparticles to the unmasked regions, yielding discretely patterned features within the binary film.;Finally, fluid flow and pattern formation during evaporative lithographic patterning of non-aqueous colloidal films differ significantly from their aqueous counterparts. Evaporative cooling creates surface tension gradients that induce recirculating flows and a majority of the particles are deposited at the interface between recirculating flow cells in the non-evaporating, masked regions. Above a critical colloid volume fraction, recirculating flows are suppressed, and a majority of the particles are convected to the regions of higher evaporative flux. |
