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fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including '' aerodynamics'' (the study of air and other gases in motion) ...
, drop impact occurs when a
drop Drop, DROP, drops or DROPS may refer to: * Drop (liquid) or droplet, a small volume of liquid ** Eye drops, saline (sometimes mydriatic) drops used as medication for the eyes * Drop (unit), a unit of measure of volume * Falling (physics), allowi ...
of
liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, an ...
strikes a
solid Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structura ...
or liquid surface. The resulting outcome depends on the properties of the drop, the surface, and the surrounding
fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shea ...
, which is most commonly a
gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
.


On a dry solid surface

When a liquid drop strikes a dry solid surface, it generally spreads on the surface, and then will retract if the impact is energetic enough to cause the drop to spread out more than it would generally spread due to its static receding contact angle. The specific outcome of the impact depends mostly upon the drop size, velocity, surface tension,
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...
, and also upon the surface roughness and the
contact angle The contact angle is the angle, conventionally measured through the liquid, where a liquid– vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, ...
between the drop and the surface.Rioboo, Romain, Cameron Tropea, and Marco Marengo. "Outcomes from a drop impact on solid surfaces." Atomization and Sprays 11.2 (2001) Droplet impact parameters such as contact time and impact regime can be modified and controlled by different passive and active methods.


Summary of possible outcomes

* "Deposition" is said to occur when the drop spreads on the surface at impact and remains attached to the surface during the entire impact process without breaking up. This outcome is representative of impact of small, low-velocity drops onto smooth
wetting Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with ...
surfaces. * The "prompt splash" outcome occurs when the drop strikes a rough surface, and is characterized by the generation of droplets at the contact line (where solid, gas, and liquid meet) at the beginning of the process of spreading of the drop on the surface, when the liquid has a high outward velocity. * At reduced surface tension, the liquid layer can detach from the wall, resulting in a "corona splash".Yarin, A. L. "Drop impact dynamics: splashing, spreading, receding, bouncing…." Annu. Rev. Fluid Mech. 38 (2006): 159-192 * On a wetting surface, "receding breakup" can occur as the liquid retracts from its maximum spreading radius, due to the fact that the contact angle decreases during retraction, causing some drops to be left behind by the receding drop. On superhydrophobic surfaces, the retracting drop can break up into a number of fingers which are each capable of further breakup, likely due to capillary instability. Such satellite droplets have been observed to break off from the impacting drop both during the spreading and retracting phases.Tsai, Peichun, et al. "Drop impact upon micro-and nanostructured superhydrophobic surfaces." Langmuir 25.20 (2009): 12293-12298 * "Rebound" and "partial rebound" outcomes can occur when a drop recedes after impact. As the drop recedes to the impact point, the
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its a ...
of the collapsing drop causes the liquid to squeeze upward, forming a vertical liquid column. The case where drop stays partially on the surface but launches one or more drops at its top is known as partial rebound, whereas the case where the entire drop leaves the solid surface due to this upward motion is known as complete rebound. The difference between rebound and partial rebound is caused by the receding contact angle of the drop on the surface. For low values a partial rebound occurs, while for high values a complete rebound occurs (assuming that the drop recedes with enough kinetic energy).


On superhydrophobic surfaces


Small drop deformation

On superhydrophobic surfaces, liquid drops are observed to bounce off of the solid surface. Richard and Quéré showed that a small liquid drop was able to bounce off of a solid surface over 20 times before coming to rest.Richard, D., and D. Quéré. "Bouncing water drops." EPL 50.6 (2000): 769 Of particular interest is the length of time that the drop remains in contact with the solid surface. This is important in applications such as heat transfer and aircraft icing. To find a relationship between drop size and contact time for low
Weber number The Weber number (We) is a dimensionless number in fluid mechanics that is often useful in analysing fluid flows where there is an interface between two different fluids, especially for multiphase flows with strongly curved surfaces. It is named ...
impacts (We << 1) on superhydrophobic surfaces (which experience little deformation), a simple balance between inertia (\rho R / \tau^2) and capillarity (\sigma/R^2) can be used,Richard, Denis, Christophe Clanet, and David Quéré. "Surface phenomena: Contact time of a bouncing drop." Nature 417.6891 (2002): 811-811 as follows: \rho R / \tau^2 \propto \sigma/R^2 where \rho is the drop density, R is the drop radius, \tau is the characteristic time scale, and \sigma is the drop surface tension. This yields \tau \propto \sqrtR^. The contact time is independent of velocity in this regime. The minimum contact time for a low deformation drop (We << 1) is approximated by the lowest-order oscillation period for a spherical drop., giving the characteristic time a prefactor of approximately 2.2.Bird, James C., et al. "Reducing the contact time of a bouncing drop." Nature 503.7476 (2013): 385-388 For large-deformation drops (We > 1), similar contact times are seen even though dynamics of impact are different, as discussed below. If the droplet is split into multiple droplets, the contact time is reduced. By creating tapered surfaces with large spacing, the impacting droplet will exhibit the counterintuitive pancake bouncing, characterized by the droplet bouncing off at the end of spreading without retraction, resulting in ~80% contact time reduction.Yahua Liu, Lisa Moevius, Xinpeng Xu,Tiezheng Qian, Julia M Yeomans, Zuankai Wang. "Pancake bouncing on superhydrophobic surfaces." Nature Physics, 10, 515-519 (2014)


Significant drop deformation

As the Weber number increases, the drop deformation upon impact also increases. The drop deformation pattern can be split up into regimes based on the Weber number. * At We << 1, there is not significant deformation. * For We on the order of 1, the drop experiences significant deformation, and flattens somewhat on the surface. * When We ~ 4, waves form on the drop. * When We ~ 18, satellite droplet(s) break off of the drop, which is now an elongated vertical column. * For large We (for which the magnitude depends on the specific surface structure), many satellite drops break off during spreading and/or retraction of the drop.


On a wet solid surface

When a liquid drop strikes a wet solid surface (a surface covered with a thin layer of liquid that exceeds the height of surface roughness), either spreading or splashing will occur. If the velocity is below a critical value, the liquid will spread on the surface, similar to deposition described above. If the velocity exceeds the critical velocity, splashing will occur and shock wave can be generated. Splashing on thin fluid films occurs in the form of a corona, similar to that seen for dry solid surfaces. Under proper conditions, droplet hitting a liquid interface can also display a superhydrophobic-like bouncing, characterized by the contact time, spreading dynamics and restitution coefficient independent of the underlying liquid properties.Chonglei Hao, Jing Li, Yuan Liu, Xiaofeng Zhou, Yahua Liu, Rong Liu, Lufeng Che, Wenzhong Zhou, Dong Sun, Lawrence Li, Lei Xu, Zuankai Wang. "Superhydrophobic-like tunable droplet bouncing on slippery liquid interfaces." Nature Communications,


On a liquid surface

When a liquid drop strikes the surface of a liquid reservoir, it will either float, bounce, coalesce with the reservoir, or splash.Rein, Martin. "Phenomena of liquid drop impact on solid and liquid surfaces." Fluid Dynamics Research 12.2 (1993): 61-93 In the case of floating, a drop will float on the surface for several seconds. Cleanliness of the liquid surface is reportedly very important in the ability of drops to float. Drop bouncing can occur on perturbed liquid surfaces. If the drop is able to rupture a thin film of gas separating it from the liquid reservoir, it can coalesce. Finally, higher
Weber number The Weber number (We) is a dimensionless number in fluid mechanics that is often useful in analysing fluid flows where there is an interface between two different fluids, especially for multiphase flows with strongly curved surfaces. It is named ...
drop impacts (with greater energy) produce splashing. In the splashing regime, the striking drop creates a crater in the fluid surface, followed by a crown around the crater. Additionally, a central jet, called the Rayleigh jet or Worthington jet, protrudes from the center of the crater. If the impact energy is high enough, the jet rises to the point where it pinches off, sending one or more droplets upward out of the surface.


See also

*
Splash (fluid mechanics) In fluid mechanics, a splash is a sudden disturbance to the otherwise quiescent free surface of a liquid (usually water). The disturbance is typically caused by a solid object suddenly hitting the surface, although splashes can occur in whic ...


References

{{reflist Fluid dynamics