The drop, exposed to an air flow parallel to the substrate, starts to dislodge when the air velocity reaches some threshold value, which depends on the substrate wetting properties and drop volume. In this study the critical air velocity is measured for different drop volumes, on substrates of various wettabilities. The substrate initial temperatures varied between the normal room temperature (24.5∘C) and subfreezing temperatures (-5∘C and -1∘C). The physics of the drop did not change at the subfreezing temperatures of the substrates, which clearly indicates that the drop does not freeze and remains liquid for a relatively long time. During this time solidification is not initiated, neither by the air flow nor by mechanical disturbances. An approximate theoretical model is proposed that allows estimation of the aerodynamic forces acting on the sessile drop. The model is valid for the case when the drop height is of the same order as the thickness of the viscous boundary in the airflow, but the inertial effects are still dominant. Such a situation, relevant to many practical applications, was never modeled before. The theoretical predictions for the critical velocity of drop dislodging agree well with the experimental data for both room temperature and lower temperatures of the substrates.

It is well-known that solidification front of a supercooled liquid is unstable; consequently, this instability leads to the appearance of an array of dendrites of sub-micron diameter. The shape and the velocity of the dendrite propagation are determined by the thermodynamic properties of the liquid and solid phases, including interfacial energy as well as the initial temperatures of both. Accordingly, the numerical simulation of solidification process is a rather challenging problem which requires an accurate prediction of high temperature gradients near the moving solidification front. In this study a relevant level set formulation has been developed enabling correct determination of the position and the curvature of the liquid/ solid interface. At this interface a Dirichlet boundary condition for the temperature field is imposed by applying a ghost-face method. For the purpose of updating the level set function and optimizing computing time a narrow-band around the interface is introduced. Within this band, whose width is temporally adjusted to the maximum curvature of the interface, the normal-to-interface velocity is appropriately expanded. The computational model is firstly validated along with the analytical solution of stable freezing. The tip velocity of dendritic patterns (pertinent to unstable freezing) is investigated by performing two-dimensional simulations. The computational results exhibit excellent qualitative and quantitative agreement with the marginal stability theory as well as with the available experiments in the heat-diffusion-dominated region.

a b s t r a c tTwo relevant computatio nal models, one relying on a Level-Set approach, the other one on a Volume-ofFluid tracking procedure with piecewise linear inter face reconstruction, are comparatively assessed in terms of their capability to simulate crystallizatio n of supercooled water. The models are preliminary validated by computing a one-dimensional freezing front propagation for which an analytic solution exists. Afterwards, the tip velocity of two-dimensional dendrites growing in supercooled water is determined and compared with corresponding experimental results and theoretical predictions in the range of supercooling between 1 K and 30 K. Present modeling results following closely both the underlying theory and experimen tal findings show very good mutual agreemen t.

A sessile water drop exposed to an air flow will shed if the adhesion is overcome by the external aerodynamic forces on the drop. In this study, shedding of water drops were investigated under icing conditions, on surfaces with different wettabilities, from hydrophilic to superhydrophobic. A wind tunnel was used for experiments in a temperature range between -8 and 24.5 °C. Results indicate that the temperature has a major influence on the incipient motion of drop shedding. The critical air velocity (U(c)) at which a drop first starts to shed generally increases under icing conditions, indicating an increase in the adhesion force. The contact angle hysteresis (CAH) and the drop base length (L(b)) are found to be the controlling factors for adhesion. A correlation was also developed to deduce the drag coefficient, C(D) for the drop. It was found that C(D) can decrease under icing conditions. In general, a lower C(D) and higher adhesion together lead to a higher critical air velocity. However, there are systems such as water on Teflon for which the critical air velocity remains practically unaffected by temperature because of similar adhesion and C(D) values, at all temperatures tested.

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