Session H6: Drops VI: Thermal Effects

Drop impact on superheated surfaces

We let millimeter droplets impact on superheated smooth surfaces. Depending on the impact velocity and the surface temperature, the droplet either immediately boils under spray formation when it contacts the surface (``wet boiling''), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back (``gentle film boiling''), or even breaks up in the middle during that process so that it basically immediately vaporizes (``violent film boiling''). We experimentally determine the phase diagram and qualitatively account for the transitions between the different regimes.   

Boiling water droplet behavior impinging on heated liquid metal surface

We observed boiling behavior of a water drop impinging on a heated liquid metal surface. By using liquid metal as impinged surface, we are able to observe boiling behavior of a water drop impinging on perfectly smooth surface. As a result of experiment, we found that a water drop starts boiling with the time delay after impinging and it changes by Weber number and initial surface temperature. And we also found that there are three factors which induce the direct contact of a water drop and surface by shooting in a high frame rate.   

Manipulating Leidenfrost temperature with surface texture

When a drop contacts a surface that is at a sufficiently high temperature, the drop can float on it's own vapor in a process known as the Leidenfrost effect. ~Although it is known that the critical temperature needed to achieve this effect depends on the properties of the drop and its vapor, often these parameters are fixed for a particular process.~Here, we demonstrate a new way to control the critical temperature through the surface structure.   

Viscous mechanism for Leidenfrost propulsion on a ratchet

An evaporating drop placed on a ratchet shaped substrate self-propels, as discovered by Linke et al. in 2006. Sublimating platelets of dry ice do the same, and we discuss a possible viscous mechanism for these motions. We report that the flow of vapor below the levitating material is rectified by the asymmetric teeth of the ratchet, in the direction of descending slopes along each tooth. As a consequence, the resulting viscous stress can entrain the material in the same direction, and we discuss the resulting self-propelling force.   

Flow near the contact line of an evaporating drop

Evaporation of water droplets with a pinned contact line is investigated in the entire range of possible contact angles (up to 150$^\circ$). We measured the evolution of the mass and the contact angle of droplets on superhydrophobic carbon nanofiber substrates. The experimental data are in quantitative agreement with a diffusion-based model, and can be collapsed onto a single universal curve for all droplet sizes and initial contact angles. However, the nature of the flow inside the drop near the contact line has remained unclear: it has been argued that the lubrication approximation does not apply in the contact-line region, and that the Marangoni effect can overcome the evaporation-driven outward flow. Here, we resolve these questions by analytically solving the Stokes flow field in a wedge geometry, imposing the evaporative flux as a boundary condition.   

Diffusion-Controlled Evaporating Perfectly wetting meniscus

The Stefan diffusion theory is often used to predict the evaporation rate from capillaries. Following Derjaguin et al (Bull. Rilem, 1965), we show here that because the simple diffusion theory does not include mass loss from the precursor film, it significantly under-estimates mass loss from small ($\mu$m-sized) capillaries. We make the following simplifying assumptions. $(a)$ Diffusion is rate limiting; as a result, liquid and vapor at the interface are in local thermodynamic equilibrium. $(b)$ The system is effectively isothermal; though evaporation induces liquid temperature differences, they are kinetically negligible. $(c)$ Transport of the evaporating molecules is by axial-diffusion only. At a given axial location $x$, the total mass flow $\dot{m}$ is the sum of the flow within the thin liquid film and that occurring by diffusion within the gas $-\dot{m} = \frac{h^3}{3\nu}\frac{dp_\ell}{dx} + \frac{\rho_sD} {\rho RT_w}(a-h)\frac{dp_\ell}{dx} = constant$ where $\nu$ is the kinematic viscosity, $R$ the gas constant, $D$ the diffusion coefficient, $\rho$ the liquid density, $T_w$ the wall temperature, $2a$ the channel gap thickness, and $\rho_s$ the saturation density. The interface shape $h(x)$ is related to the liquid pressure at the interface $p_\ell(x)$ by the normal stress equation $P_a - p_\ell = \gamma\frac{d^2h}{dx^2} + \frac{A}{h^3}$ where $P_a$ is the total gas pressure and $A$ the Hammaker constant. Our model differs from that of Derjaguin et al because including surface tension $\gamma$ allows us to calculate the shape of the whole meniscus, and so to relate our results for mass loss to that given by standard theory.   

Planar Jumping-Drop Thermal Diodes

Phase-change thermal diodes transport heat asymmetrically with a large rectification coefficient unmatched by their solid-state counterparts, but are limited by either the gravitational orientation or one-dimensional configuration. We report a planar phase-change diode scalable to large areas with an orientation-independent diodicity of up to 100, in which water/vapor is enclosed by parallel superhydrophobic and superhydrophilic plates. The thermal rectification is enabled by spontaneously jumping dropwise condensate which only occurs when the superhydrophobic surface is colder than the superhydrophilic surface. Our jumping-drop thermal diode is expected to be particularly useful for the thermal protection of planar electronic components and the thermal regulation of large-area energy harvesting systems.   

Droplet manipulation using light-induced thermal flows on an amorphous silicon thin film

We present a droplet-manipulation method using light-induced thermal flows in oils. The flows are originated from Marangoni and buoyancy effects due to temperature gradient, generated by the adsorption of light in an amorphous silicon thin film. The effects of the types and thickness of the oil on the flows are explored experimentally and numerically. Using this method, we can manipulate a droplet including cells in an extremely simple system without the damage on cell viability.   

Pattern Formation in Drying Drops of Polystyrene/Water nanofluids

We study the pattern formation and the evaporation dynamics of drying drops of polystyrene/water based nanofluids with concentrations ranging from 0.01{\%} to 6{\%}. Cracks formation is evidenced to depend on the nanoparticles concentration. The dynamics of evaporation is recorded using an electronic balance with an accuracy of 10$\mu $g. A top view recording enables to analyze the pattern formation in relation with the mass evolution. We determine several key parameters such as the time of evaporation, the wetting diameter, the final solid deposition diameter, the number and the spacing of the cracks. We evidence a ring formation above a critical concentration. We evidenced by change of the surrounding humidity in the range of 10 to 90{\%} that this pattern remains constant. The pattern formation is influenced by the liquid phase evaporation dynamics but only depends on the concentration in nanoparticles. These results are of great interest regarding the formation of droplets in several areas such as inkjet printing, pharmacology...   

Numerical study of thermocapillary instabilities in evaporating annular pools and sessile droplets

We investigate thermocapillary flows due to temperature-induced surface tension gradients in annular liquid pools via full two-phase direct numerical simulations in 3D. Phase-change, interface deformation and wettability phenomena are taken into consideration by using a variant of the volume-of-fluid method. The simulation results are validated against experiments (Schwabe et al. 2003 {\&} Riley et al. 1998) and theory (Smith {\&} Davis 1983). The transient results show the evolution of the flow towards an oscillatory state characterized by interfacial hydrothermal waves (HTWs). We present the effects of non-uniform evaporation fluxes and the liquid depths on the linear and non-linear development of these thermocapillary instabilities. The influence on bulk flows, surface temperature patterns and interface deformations are also shown. We finally introduce spontaneously self-excited HTWs in evaporating sessile droplets simulated using novel numerical methods and compare the results against analytical models and experiments.