Melting of Colloidal Crystal Films

We study melting mechanisms in single and polycrystalline colloidal films composed of diameter-tunable microgel spheres with short-ranged repulsive interactions and confined between two glass walls. Thick films ($>4$ layers), thin-films ($\le 4$ layers), and monolayers exhibit different melting behaviors. Thick films melt from grain boundaries in polycrystalline solid films and from film-wall interfaces in single-crystal films; a liquid-solid coexistence regime is observed in thick films but vanishes at a critical thickness of 4 layers. Thin solid films (2 to 4 layers) melt into the liquid phase in one step from both grain boundaries and from within crystalline domains. Monolayers melt in two steps with a middle hexatic phase.

Figure 1

(a) A 12-layer thick polycrystalline solid melts from grain boundaries. (b) A 5-layer thick polycrystalline solid melts from both grain boundaries and from within the crystalline domains. Left loop: a liquid “lake.” Right hand side dashed line: a liquid “strip” between two domains. (The liquid domain on the right extends out of the image.) (c) Three-dimensional schematic of (b). (d) A 4-layer polycrystalline solid melts from random positions within the sample. Small defects do not nucleate into large liquid clusters and domains. (e) The liquid-solid coexistence regime (expressed as the temperature window interval $\Delta T$ or packing fraction interval $\Delta \varphi$ for coexistence) decreases with the film thickness.

Figure 2

2▵ crystal melting. Here, the results are derived from one of the two surface layers. (a) Two-dimensional translational, 2D and 3D orientational order parameters: ${\psi }_T$, ${\psi }_6$, and $Q_6$, respectively, as a function of sample temperature $T$ and packing fraction $\varphi$. (b) Translational and orientational susceptibilities ${\chi }_T$ and ${\chi }_6$. (c) Orientational correlation functions $g_6(t)$ in time. (d) Orientational correlation functions $g_6(r)$ in space.