Geometry-Encoded Droplet Evaporation on Rectangular Micropillared Surfaces: From Wetting-State Transitions to Programmable Colloidal Deposit Morphology

Evaporation-driven deposition of colloidal particles on microtextured surfaces offers a versatile route to material patterning for technologically critical processes, including precision inkjet printing, functional coating, biosensing, and directed colloidal self-assembly; yet predictive control over deposit geometry remains challenging due to the coupled interplay of wetting-state transitions, contact-line (CL) dynamics, and surface architecture. Here, we present a systematic experimental investigation of pure water and monodispersed colloidal sessile droplet evaporation on rectangular micropillared polydimethylsiloxane (PDMS) surfaces, wherein pitch (60 and 80 μm), in-plane orientation (0-0 and 0-90), lattice arrangement (inline and staggered), and substrate temperature (22-80 °C) are independently varied. The Cassie-Baxter to Wenzel transition emerges as the decisive geometry-programmable determinant of deposition footprint size, enabling equivalent diameters of 0.55-0.98 mm by modulating the micropillar design and substrate temperature. For representative micropillared surfaces with P = 60 μm, classical destabilization criteria are not satisfied; instead, evaporation-induced vapour confinement promoting wetting-film growth and sidewall advancement is proposed as the physically plausible mechanism dictating transition. For P = 80 μm, transition onset is governed by pressure-driven capillary entry. Post-transition CL evolution in Wenzel state is shown, in a hitherto unreported mechanistic framework, to be governed by geometry-imposed constraints on CL motion, wherein interpillar gap geometry and lattice symmetry drive anisotropic meniscus-mediated spreading and discrete micropillar-row anchoring. These dynamics prescribe octagonal, rectangular, hexagonal, and square morphologies with anisotropy factors of 1.10-1.50. Colloidal particles suppress transition-induced spreading through enhanced pinning, whereas substrate heating promotes meniscus-mediated secondary spreading, enabling coupled control of deposit size and shape. These findings establish a physically interpretable, multidimensional design space for programmable evaporation-driven microscale deposition, advancing the rational engineering of surface-texture-directed patterning for these applications.