Fig. 4.

Evaporative isochoric thickening. (A) SEM and (B) AFM micrographs of nanoparticle-based structures formed after the ejection of droplets for 60, 70, 120, and 160 ms. (C) AFM height profiles (blue line) taken along the indicated lines in B demonstrate that all structures but the last (i.e., at 160 ms) have the shape of an equally wide spherical cap (red lines) with a contact angle, increasing with ejection duration. In (D) fitting of the last structure is only performed at its basis. This lower part represents the shape of the original sessile droplet, which in the course of isochoric thickening has been subject to sudden crystallization of its nanoparticle content. Owing to a change in wettability, a nanopillar starts to grow on top of this crystallized pedestal. Whereas at 160 ms in D the nanopillar is merely a nipple, it substantially grows when ejecting for an extended period of 500 ms (E). The applied voltage in all experiments was 200 V.