Interfacially arrested melting in thin films: capillarity-driven suspension of phase transitions

TL;DR

The paper addresses whether melting, a canonical first-order transition, can be thermodynamically arrested in confined thin-film geometries. Using molecularly thin alkane films with trapped liquid droplets, the authors reveal a capillarity-driven mechanism in which interfacial energy competes with bulk melting enthalpy, yielding a stationary partially melted state controlled by $\Delta T \cdot h$ and independent of droplet size. They derive and validate the key relation $\cos \theta_a = \cos \theta_0 + \frac{\Delta S \cdot \Delta T \cdot h}{\gamma_{lv}}$, and demonstrate master-curve scaling and intrinsic thermodynamic amplification of temperature variations via optical (Newton-ring) readouts. The results point to a general, material-agnostic pathway to stabilize partial phase transitions in confined thin films, with potential applications in high-resolution thermometry and pattern control in micro/nano-fabrication.

Abstract

Melting is typically viewed as a bulk first-order phase transition that proceeds once nucleation barriers are overcome. Here we demonstrate an interfacially arrested melting regime in molecularly thin crystalline films, where large liquid droplets remain stably trapped well above the bulk melting temperature. Using long-chain alkane films as a model system, we show that melting is suspended by the competition between bulk melting enthalpy and interfacial energy costs associated with capillary confinement. The arrested state is governed by a single control parameter, the product of temperature offset and film thickness, and is independent of droplet size. As a consequence, small temperature variations produce pronounced and reversible changes in droplet morphology, enabling intrinsic thermodynamic amplification of thermal signals. These results reveal a general mechanism by which interfacial constraints can arrest first-order phase transitions in thin films.

Figures (5)

• Figure 1: Schematic illustration of the experimental setup and data analysis procedure. Optical interference between light reflected from the alkane–air and silicon–silica interfaces enhances the vertical resolution (a), providing sufficient contrast to resolve an alkane monolayer with a thickness of approximately $4,\mathrm{nm}$ (b). To extract droplet profiles, raw image (c) is first processed using a high-pass filter (d). The positions of the peaks and valleys of the resulting Newton rings are then identified and converted into height information (e). Owing to the static and axisymmetric nature of the droplets, the reconstructed profiles are fitted with a circular cap to determine the apparent contact angle (f).
• Figure 2: Reversible expansion and retraction of droplets with temperature. (a) Two droplets embedded in alkane films flatten as temperature increases. The response is more pronounced for the droplet trapped in a three-layer film (droplet $\#3$) than for that in a two-layer film (droplet $\#2$), whereas a reference droplet residing on top of the solid film shows no measurable change in apparent contact angle $\theta_a$. The edge of the three-layer film is marked by a thin line. (b) The morphological response is fully reversible upon temperature cycling. (c) As predicted by the energetic analysis, $\cos\theta_a$ varies linearly with the temperature offset.
• Figure 3: Normalized free energy $\bar{G}_{\mathrm{total}} = G_{\mathrm{total}}/(\gamma_{lv} r_0^2)$ as a function of the apparent contact angle $\theta_a$. When melting proceeds with the liquid front pinned at the solid boundary, $\theta_a$ decreases from the Young contact angle $\theta_0=17^\circ$ toward zero. Film thickness is fixed at $4\,\mathrm{nm}$, and curves correspond to increasing $\Delta T$ from $0.1\,\mathrm{K}$ to $0.9\,\mathrm{K}$. In the arrested melting regime, variations in the control parameter $\Delta T \cdot h$ modify the balance between bulk and interfacial energies, leading to a continuous shift of the free-energy minimum (red dots). Inset: schematic of a droplet embedded in a solid film, illustrating the geometric constraints and relevant parameters of the energetic model.
• Figure 4: Collapse of the apparent contact angle data when plotted as $\cos\theta_a - \cos\theta_0$ versus the combined control parameter $\Delta T \cdot h$. Data from films of different thickness fall onto a single master curve, confirming that arrested melting is governed by the product $\Delta T \cdot h$, independent of droplet size.
• Figure 5: Enhanced thermodynamic amplification in multilayer alkane films. Droplets embedded in four-layer (a1 to 3) and six-layer (b1 to 3) films exhibit pronounced displacement of Newton rings under temperature variations well below $0.1^\circ$C, indicating strong thickness-dependent sensitivity.