We investigated the evaporative crystallization of aqueous glycine sessile droplets on hydrophilic glass, hydrophobic Teflon surfaces, and hydrophobic Teflon surfaces, where the contact angle is manipulated dynamically with electrowetting. Microscopy experiments and analytical ch
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We investigated the evaporative crystallization of aqueous glycine sessile droplets on hydrophilic glass, hydrophobic Teflon surfaces, and hydrophobic Teflon surfaces, where the contact angle is manipulated dynamically with electrowetting. Microscopy experiments and analytical characterization revealed that the size, morphology, and polymorphic form (α, β, and γ) of the glycine crystals are influenced by the surface wettability as well as the amplitude and frequency of electrowetting. On a hydrophilic glass surface, a coffee-stain-shaped residue composed of a mixture of bipyramidal α and needle-like β crystals was observed. On a hydrophobic Teflon surface, the droplets evaporated with minimum contact line pinning, producing hemispherical residue shapes, and bipyramidal α crystals smaller than 100 μm were formed. On a Teflon surface with electrowetting, glycine could be manipulated to crystallize into distinct polymorphic forms (β and γ) and residue shapes not observed on hydrophilic glass and hydrophobic Teflon surfaces. The frequency and amplitude of electrowetting were optimized to produce single large crystals. We observed the highest chance of producing single-millimeter-scale crystals at a frequency of 1 kHz and a voltage amplitude of 80 Vrms. We attribute this observation to a combination of nucleation at lower bulk supersaturation compared to the experiment on Teflon surfaces and electrowetting-induced mixing most prominent at 1 kHz. Our results highlight the opportunities arising from the dynamic manipulation of surface wettability