The developments in nanocoating and nanospray technology have resulted in the increasing importance of the impact of micro-/nanoscale liquid droplets on solid surface. In this paper, the impact of a nanodroplet on a flat solid surface is examined using molecular dynamics simulations. The impact velocity ranges from 58 m/s to 1044 m/s, in accordance with the Weber number ranging from 0.62 to 200.02 and the Reynolds number ranging from 0.89 to 16.14. The obtained maximum spreading factors are compared with previous models in the literature. The predicted results from the previous models largely deviate from our simulation results, with mean relative errors up to 58.12%. The estimated viscous dissipation is refined to present a modified theoretical model, which reduces the mean relative error to 15.12% in predicting the maximum spreading factor for cases of nanodroplet impact. C 2015 AIP Publishing LLC. [http://dx.
The normal impinging of nanoscale water droplets on the solid surface is investigated through molecular dynamics simulations. A wide regime of impinging from spreading to breakup is studied. The overestimations of dissipation and surface free energy in literature are modified with a more accurate assumption on flow fields. The refined model fits well with the simulation results by introducing the linear distribution of radial velocity gradient. Two modes of breakup are observed during the nanodroplet impinging on the surface: (1) touch-bottom of the surface of the liquid film and (2) propagation of finger-like projections on the flow frontier. The touch-bottom breakup is possibly the dominant mode in cases with large We and small Re. The criterion is proposed to be that the amplitude of the capillary wave is larger than the average height of the droplets at the maximum spreading state. This criterion gives a well prediction comparing to the results obtained in molecular dynamics simulations.
Nucleation of water on solid surface can be promoted noticeably when the lattice parameter of a surface matches well with the ice structure. However, the characteristic length of the surface lattice reported is generally less than 0.5 nm and is hardly tunable. In this paper, we show that a surface with nanoscale roughness can also remarkably promote ice nucleation if the characteristic length of the surface structure matches well with the ice crystal. A series of surfaces composed of periodic grooves with same depth but different widths are constructed in molecular dynamics simulations. Water cylinders are placed on the constructed surfaces and frozen at constant undercooling. The nucleation rates of the water cylinders are calculated in the simulation using the mean first-passage time method and then used to measure the nucleation promotion ability of the surfaces. Results suggest that the nucleation behavior of the supercooled water is significantly sensitive to the width of the groove. When the width of the groove matches well with the specific lengths of the ice crystal structure, the nucleation can be promoted remarkably. If the width does not match with the ice crystal, this kind of promotion disappears and the nucleation rate is even smaller than that on the smooth surface. Simulations also indicate that even when water molecules are adsorbed onto the surface structure in high-humidity environment, the solid surface can provide promising anti-icing ability as long as the characteristic length of the surface structure is carefully designed to avoid geometric match.
We investigate the impact behaviors of nanoscale polymer droplets on a solid surface via molecular dynamics (MD) simulations. The maximum spreading factor is focused on understanding the energy dissipation mechanism during impact. Our simulations show that the macroscale model for blood droplets and the nanoscale models for water droplets cannot capture the simulated maximum spreading factor of nanoscale polymer droplets. We demonstrate that viscous dissipation for nanoscale polymer droplets stems from the velocity gradients in both the impact and the spreading direction, whereas for macroscale blood droplets and nanoscale water droplets, only the velocity gradient in the impact direction contributes to it. With the consideration of different dissipation mechanism, we propose a modified expression of viscous dissipation and develop a new model to predict the maximum spreading factor of nanoscale polymer droplets. By comparing the model predictions with MD simulations, we show that the new model can capture more precisely the simulated maximum spreading factor for nanoscale polymer droplets with chain length ranging from 10 to 100. The present simulations and developed model can provide useful insights into the mechanism of nanoscale polymer droplets impacting surfaces.
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