Quantitative high-speed imaging of ice-crystal impact, melting, and amalgamation into ice layers on an ice-cloud-immersed heated surface

Using high-speed imaging and a custom image-processing framework, we reveal a non-monotonic effect of the heating ratio, Γ-which represents the ratio of imposed substrate heat flux to the thermal loading of the incoming ice cloud-on the late-stage evolution of an initiating ice accretion forming on a heated stagnating substrate, during which the surface transitions from dry to a fully developed quasi-one-dimensional ice layer. Specifically, increasing Γ initially accelerates areal spreading before further increases suppress it. This non-monotonic behavior is directly tied to a transition in accretion morphology with increasing heating: for Γ ≲0.1, increases in heating shift the late-stage morphology away from predominantly solid ice crystals toward sprawling, relatively immobile, slush-like amalgamations that spread through the formation of liquid bridges. The extensive footprint and low mobility of these structures accelerate areal coverage and structure growth while simultaneously depleting structure number density through amalgamation. Conversely, for Γ ≳0.1, increasing heating promotes the formation of large liquid droplets that coalesce into areally compact structures, thereby suppressing areal coverage and structure growth while reducing the depletion rate of structure number density. These collective dynamics emerge only after the surface saturates with accreted structures; prior to saturation, accretion evolution remains governed primarily by isolated crystal impacts, with heating promoting adhesion but exerting little influence on areal coverage or structure growth. These findings demonstrate that heating influences initiation-phase ice-crystal accretion not only through its effect on individual crystal adhesion, but also through its control of the collective morphological evolution and interactions of accreted structures that ultimately govern the late-stage spreading of the nascent ice layer.