Session R36: Drops: Impacts: Heat Transfer and Leidenfrost

Heat transfer and phase change in an impinging droplet

Non isothermal droplet impact on solid surfaces has several industrial applications such as spray cooling and 3D printing. Impinging of a droplet on a surface involves an initial phase of spreading followed by a subsequent return to the equilibrium shape. Thermal energy exchanged within the droplet fluid as well as between liquid/solid during the impact has been studied using an ultra high speed infrared camera. Variable parameters in the experiment include droplet temperature and kinetic energy of the droplet during the impact. The evolution of droplet shape viewed by IR camera is similar to what previously observed by high speed photography. The thermal map of droplet over time in these experiments agrees with previously reported numerical simulation. In addition, spacial and temporal temperature variations of liquid droplets on a surface as they solidify are presented. IR camera provides an accurate temperature diagram as the phase change occurs, which is essential for understanding the physics of 3D printing.   

Theoretical and Experimental Analyses of Molten Droplet Impact on Cold Substrates

Spreading of liquid drop on cold solid substrates is a complicated problem that involves heat transfer, fluid dynamics, and phase change physics with the combination of complex wetting behavior of contact line. Many researchers are trying to obtain the final shape of the droplet or in other words the contact angle and radius of the drop after the solidification is complete. Understanding the physics behind the non-isothermal spreading of droplet is of utmost importance owing to its broad applications in diverse areas of industry. This work mainly focuses on obtaining important physical parameters involved in the process of spreading of molten droplets as well as controlling the post-solidification geometry of droplets. A complete set of experimental study is performed that shows the final radius in the case of free fall of droplet under high impact velocity is independent of the initial condition of the impact including the impact velocity and temperature gradients. The analytical modeling of the problem also verifies the accuracy of these results.   

Impact of Metal Droplets: A Numerical Approach to Solidification

Layer-wise deposition of material to produce complex products is a subject of increasing technological relevance. Subsequent deposition of droplets is one of the possible 3d printing technologies to accomplish this. The shape of the solidified droplet is crucial for product quality. We employ the volume-of-fluid method (in the form of the open-source code Gerris) to study liquid metal (in particular tin) droplet impact. Heat transfer has been implemented based on the enthalpy approach for the liquid-solid phase. Solidification is modeled by adding a sink term to the momentum equations, reducing Navier-Stokes to Darcy's law for high solid fraction. Good agreement is found when validating the results against experimental data \footnote{S.D.Aziz, S.Chandra, Int. J. of Heat and Mass Trans. 43 (2000)}. We then map out a phase diagram in which we distinguish between solidification behavior based on Weber and Stefan number. In an intermediate impact regime impact, solidification due to a retracting phase occurs. In this regime the maximum spreading diameter almost exclusively depends on Weber number. Droplet shape oscillations lead to a broad variation of the morphology of the solidified droplet and determine the final droplet height.   

The impact of a Leidenfrost drop on a spoked surface texture

Liquid drops can bounce when they impact non‐wetting surfaces. Recently, studies have demonstrated that the time that the bouncing drop contacts a superhydrophobic surface can be reduced by incorporating ridged macrotextures on the surface. Yet the existing models aimed at explaining this phenomenon offer incompatible predictions of the contact time when a drop impacts multiple intersecting macrotextures, or spokes. Furthermore, it is unclear whether the effects of the macrotexture on the drop hydrodynamics extend to non-wetting surfaces in which direct contact is avoided by a thin vapor layer. Here we demonstrate that the phenomenon observed for macrotextured, superhydrophobic surfaces extends to macrotextured, wettable surfaces above the Leidenfrost temperature. We show that the number of droplets and overall residence time both depend on the number of intersecting spokes. Finally, we compare and contrast our results with mechanistic models to rationalize various elements of the phenomenon.   

An experimental study of the dynamic Leidenfrost phenomenon

Complete separation between an impacting droplet and a superheated surface can be achieved if the surface temperature is sufficiently high causing spontaneous generation of a vapor layer under the droplet. The transition to such vapor-induced separation, or Leidenfrost regime, depends on numerous parameters such as materials properties and the impact conditions including the impact velocity and surface temperature. Here we provide detailed experimental observations of several distinct impact dynamics at the Leidenfrost transition in order to understand the physical mechanism of such transition. We focus on the liquid-solid interface to identify necessary conditions for Leidenfrost transition to occur. We show that detailed and quantitative measurements of the wetted area during impact may lead to a physical understanding of the Leidenfrost phenomenon.   

Animating Impacting Spheres with the Elastic Leidenfrost Effect

Liquid droplets impacting on hot surfaces above the Leidenfrost temperature can squeeze out the vapor layer and enter the contact boiling regime. What happens to soft but vaporizable solids, such as hydrogel spheres, under such conditions? I will show how this combination leads to sustained bouncing dynamics. The key physics is the coupling between the sphere's elastic deformations and vaporization. Beyond being a new facet of the Leidenfrost effect, this phenomenon promises to be useful in fields such as fluid dynamics, microfluidics, and active matter.   

Thermo-responsive droplet deposition and solidification

The spreading of a thermo-responsive droplet on a heated surface is studied. The spatio-temporal pattern~of gel formation within the droplet is visualized using a new experimental method based on spectral~domain optical coherence tomography. The method relies on a collective motion of sub-micron buoyant~particles inside the droplet. The mechanisms that lead to the arrest of the spreading droplet are~explored.~The importance of evaporation-induced gel formation and heat conduction~through surrounding air are highlighted.~The proposed experimental technique can potentially be used to analyze the~solidification of different fluids such as molten waxes. Thermo-responsivity is demonstrated to provide~an effective control over the final shape of the droplet.