Soft PDMS-mediated formation of bovine serum albumin 3D coffee stain structures for reversible hydrophilic patterning

Abstract

The evaporation of sessile drops containing non-volatile solutes produces a 2D ring-like deposition of particles at the contact line known as the coffee stain effect. In addition to 2D ring structures, evaporating protein drops can also produce 3D dome-shaped coffee stain structures (CSS) as well. In this study, a new 3D CSS, the buckling saddle-shaped film is reported when a bovine serum albumin (BSA) drop dries over a soft polydimethylsiloxane (PDMS) substrate. By systematically varying substrate wettability, elasticity, and protein concentrations, we have found that the saddle CSSs are only formed when an intermediate-concentration (1–10% w/v) BSA drop evaporates over a soft (<10 MPa) and hydrophobic substrate. Time-resolved epi-fluorescence microscopy of a BSA drop doped with fluorescently labeled BSA on a PDMS substrate reveals a new formation mechanism of the saddle CSS, which originated from subtle interplays of time scales of solvent evaporation, solute dispersion, and “skin” formation. Differing from the dome CSS, saddle CSS is formed by rapid “drainage” of the solution film between interfacial “skin” and substrate that leads to the asymmetrical distribution of BSA along the contract line and subsequently causes the interfacial “skin” to buckle as it dehydrates. We further discover that the abovementioned formation mechanism deposits a microscopically smooth thin protein film on the substrate underneath the drop. This discovery further inspires the proposed reversible hydrophilic patterning technique.