Evaporation driven smart patterning of microparticles on a rigid-soft composite substrate

Hypothesis: A liquid droplet on a rigid polydimethylsiloxane (PDMS) substrate exhibits a higher receding contact angle (θ_r), therefore, recedes earlier than its softer counterpart. The three-phase contact line of a suspension droplet on a composite rigid-soft PDMS substrate can be selectively tuned wherein the contact line recedes on the rigid substrate sooner and approaches toward the softer side, with microparticles eventually being deposited in the softer substrate region. Experiments: A composite PDMS substrate containing soft cores of various shapes (circular and non-circular) surrounded by rigid matrices was fabricated by employing 3D printing and soft lithography. A sessile suspension droplet containing spherical microparticles was deposited on the composite substrate and evaporated under ambient conditions. The evaporation dynamics was recorded and analyzed. Findings: Evaporation-induced patterning (in circular, triangular, and rectangular areas) with sizes ranging from microns to millimetres were obtained. For the first time, by varying the ratio of the rigid-soft regions in the PDMS substrate, we were able to obtain different deposition sizes and shapes from an identical droplet. Instead of using lithographically patterned substrate, our simple methodology by using 3D printing and soft lithography opened up a new avenue for patterning microparticles based on a rigid-soft composite substrate.