Spontaneous cracking of amorphous solid water films and the dependence on microporous structure

Abstract

Vapor-deposited, porous, amorphous, water-ice films, also called amorphous solid water (ASW), crack spontaneously during growth when the film thickness exceeds a critical value (Lc). We measured the Lc during growth of ASW films as a function of growth temperature (Tg = 10 K, 30 K, and 50 K) and deposition angle (θ = 0°, 45°, and 55°) using a quartz crystal microbalance, an optical interferometer, and an infrared spectrometer. The critical thickness, 1–5 μm under our experimental conditions, increases with Tg and θ, an indication of film porosity. We suggest that ASW films undergo tensile stress due to the mismatch between substrate adhesion and contracting forces derived from the incompletely coordinated molecules on the surfaces of the pores. We provide a model to explain the observed dependences of Lc on the Tg and θ in the context of Griffith theory and estimate the tensile strength of low-temperature ASW to be ∼25–40 MPa. Our model can be applied more generally to describe fracture of other solids with microporous structures, such as metallic or ceramic materials with voids.