Session L13: Drop Impact on Solids I

Drop impact on a solid surface at high velocity

Drop impacts are usually studied for impact velocities up to about 5 m/s. We have constructed a new facility where we can, in a controlled manner, reach much higher impact velocities up to 23 m/s. In this way we can generate Reynolds ~ 10⁵ and Weber  ~ 4  x 10⁴ . Using water/glycerin mixtures we can systematically vary the Reynolds number over three orders of magnitude, while keeping the Weber number very large, thereby reaching new corners of parameter space. Our focus is on the lamellar splashing, which we observe with up to 5 million frames per second, using a Kirana-5M video camera. The lamellar thickness reduces greatly at the highest velocities, while its jetting velocity reaches 400 m/s, or near 20 times the impact velocity. We highlight the importance of the bottom radius of curvature of the drop at first contact, especially when the ambient pressure is varied.   

Analysis of behavior of single water droplet after impingement on a flat surface of different properties

Behavior of single rain droplet after impingement on a flat surface placed horizontally was studied. Most studies concerning rain droplet impingement done so far have been aimed at the impact force measurement carried out experimentally from the mechanical, civil, and agricultural engineering point of view.. However, the precise analysis of the behavior of the thin water film spreading on the surface after collision was not performed. Hence, we conducted the test to find out how a droplet behaved in association with data collection of the water spreading speed and the radius of a water circle. In order to find out the effect of the surface property on the droplet’s behavior, the surface of a specimen on which the droplet impacted was to be of different wettability: hydrophilicity, hydrophobicity and superhydrophobicity. The amount of water dripped from the micro pipette was carefully controlled to produce the droplet with a diameter corresponding to the average size of natural rain drops. The very minute plastic particles with some micron diameter were added to water to drop for the image analysis after the test. For the image analysis, the particle tracking velocimetry was employed.   

Influence of the impact energy on the pattern of blood drip stains

The maximum spreading diameter of complex fluid droplets has been extensively studied and explained by numerous physical models. This research focuses therefore on a different aspect, the bulging outer rim observed after evaporation on the final dried pattern of blood droplets. A correlation is found between the inner diameter, the maximum outer diameter, and the impact speed. This shows how the drying mechanism of a blood drip stain is influenced by the impact energy, which induces a larger spreading diameter and thus a different redistribution of red blood cells inside the droplet. An empirical relation is established between the final dried pattern of a passive bloodstain and its impact speed, yielding a possible forensic application. Indeed, being able to relate accurately the energy of the drop with its final pattern would give a clue to investigators, as currently no such simple and accurate tool exists.   

Catapulting impacting droplets via a wettability-patterned metal mesh

A water droplet striking orthogonally a superhydrophobic metal mesh may partially penetrate the mesh if the stagnation pressure is greater than the breakthrough pressure. On the other hand, the same droplet impacting a superhydrophilic mesh tends to stick to the wires due to the high pinning force. In the present work, experimental observations of droplet impact on wettability-patterned metal meshes are presented. The droplet impacts on the contrast line separating a semi-infinite superhydrophobic domain from a superhydrophilic counterpart. It is observed that, upon impact, a part of the droplet penetrates through the superhydrophobic side, while the liquid impacting the superhydrophilic side gets pinned. This pinning phenomenon forces the rest of the droplet on the superhydrophobic side, that undergoes rebound upon impact, to be vectored toward the superhydrophilic side. For a specific range of droplet diameter and pore size of the mesh, the momentum of the vectoring portion may be so large that a part of it breaks away and is launched in a lateral direction, akin to a projectile. The physical processes responsible for such a motion are identified, and a regime map is presented that separates the different types of post-impact behaviors observed in the experiments.   

Molecular Dynamics Simulations of Homogeneous Nucleation of R1233zd(E) Refrigerant

R1233zd(E) (1-chloro-3,3,3-trifluoroprop-1-ene) is a potential alternative refrigerant for vapor compression refrigeration due to its advantages of zero ozone depletion potential (ODP), low global warming potential (GWP), non-toxic and non-flammable, etc. In this paper, molecular dynamics simulations have been employed to study the vapor-liquid nucleation processes of R1233zd(E) in vapor compression refrigeration system. The variation of energy, radial distribution functions, density and isobaric heat capacity of R1233zd(E) during the vapor-liquid nucleation processes were analyzed. The computed densities of R1233zd(E) both in vapor and liquid phases agree well with the values in literature, which verifies the reliability of our simulation method.