Nucleation at the Contact Line Observed on Nanotextured Surfaces

It has been conjectured that roughness plays a role in surface nucleation, the tendency for freezing to begin preferentially at the liquid-gas interface. Using high speed imaging, we sought evidence for freezing at the contact line on catalyst substrates with imposed characteristic length scales (texture). Length scales consistent with the critical nucleus size and with $\delta \sim \tau /\sigma$, where $\tau$ is a relevant line tension and $\sigma$ is the surface tension, range from nanometers to micrometers. It is found that nanoscale texture causes a shift in the nucleation of ice in supercooled water to the three-phase contact line, while microscale texture does not.

Figure 1

Fabricating nanoscale surface texture. Motivated by the conjectured importance of roughness to heterogeneous nucleation and the plausible range of length scales, these experiments were conducted with smooth optical fibers (a),(b), nanotextured optical fibers (c),(d), and microtextured silicon substrates (see the Supplemental Material, Fig. S2 [43]) as heterogeneous nucleation catalysts. Panels (a)–(d) were taken with a high-resolution SEM.

Figure 2

Three modes of nucleation. Top: a schematic of the droplet-fiber geometry. A $30\mu \mathrm{L}$ droplet with a contact angle of $\approx 90°$ rests on a siliconized glass slide (Hamilton Scientific) that is cooled from below [28]. An optical fiber, partially immersed within the droplet, can act as a heterogeneous nucleation catalyst. Three possibilities for nucleation then arise: on the substrate (red), on the immersed fiber (green), and at the fiber contact lines (blue). Bottom: by imaging the crystallization at 5 kHz we pinpoint the nucleation site (boxed area in film strips). Film strips here represent each of the three nucleation modes. Every 15th frame is shown resulting in a 3 ms spacing. See Supplementary Movie for more detail [43].

Figure 3

Nanotexture observed to cause a transition to surface nucleation at the contact line. Results for the three modes of nucleation, substrate, immersed fiber, fiber contact line (see Fig. 2). Left panel: spatial origin of crystallization is observed often to shift to the contact line for nanotextured (rough) fibers, but neither the smooth fiber ($r=70\mu \mathrm{m}$) nor the microtextured substrates ($2–100\mu \mathrm{m}$) yield such a shift (see the Supplemental Material, Sec. 2 [43]). Despite relatively small surface area of the nanotextured fiber, over half of freezing events are initiated there. Furthermore, despite the overwhelmingly small spatial odds, the majority of the fiber induced events originate at the contact line. Right panel: higher freezing temperatures (weaker supercooling) are observed for nucleation events at the fiber contact line as evidenced by the cumulative freezing probabilities (red curve). Broadening of the distribution accompanies fiber contact line events, as expected because of additional variability in the geometry of the nanotextured contact lines.