The role of surfactant in evaporation and deposition of bi-solvent biopolymer droplets.

Inkjet printing of biopolymer droplets is gaining popularity because of its potential applications in regenerative medicine, particularly the fabrication of tissue-regenerative scaffolds. The quality of bioprinting, which affects cellular behaviors and the subsequent tissue formation, is determined by the solvent evaporation and deposition processes of biopolymer droplets, during which instantaneous local viscosity and surface tension changes occur because of the redistribution of the biopolymer inside the drop. Such dynamics is complex and not well understood. Most biopolymer inks also contain multiple solvents of distinct evaporation rates, further complicating the system dynamics. Using high-speed interferometry, we directly observe in real time the instantaneous drop shape of inkjet-printed picoliter gelatin drops containing glycerol and water. It is observed that, for bisolvent gelatin drops with surfactants, highly viscous gelatin and glycerol accumulated near the pinned contact line at an early stage suppress the evaporation-driven outward flow and create a stagnation zone near the contact line region. Lower surface tension at the contact line, because of its high local surfactant concentration, as compared to the drop apex induces a strong Marangoni recirculation, which in conjunction with a stagnation zone in the contact line region causes the instantaneous drop shape to transition from a spherical cap to a volcano shape during evaporation and resulting in a volcano-like deposition profile. In contrast, the suppressed evaporation outward flow together with a weak Marangoni flow leads to a domelike deposition for the case without surfactant. The role of surfactant in polymer drop deposition with water-only solvent is also investigated and compared against that of bisolvent drops. For the single-solvent case, the deposition profile is found to shift from a coffee-eye shape in the presence of surfactant to a uniform deposition without surfactant. The results reveal new insight into the complex role surfactant plays during polymer drop evaporation and deposition processes.