Experimental and Model Studies of Various Size Water Droplet Impacting On a Hydrophobic Surface

Impacting droplet on a hydrophobic surface is investigated and droplet size effect on impacting properties is examined. Liquid pressure variation inside droplet is numerically simulated in the impacting and rebounding periods. Droplet motion on impacted hydrophobic surface is monitored using a high-speed recording system. We showed that predictions and high-speed data for droplet shape and geometric features appear to be almost identical in the spreading and retraction of the droplet on sample surface. Increased volume of droplet gives rise to the peak pressure enhancement in droplet liquid during impact. The maximum droplet height remains larger for large volume droplets in both spreading and retraction cycles. Increasing size of droplet enlarges the wetting diameter on the impacted surface during droplet deformation on sample surfaces. The rate of peak velocity of the spreading surface of the droplet is faster for small droplets as compared to that corresponding to large droplets. The ratio of spreading period over the retraction period of the droplet becomes small for droplets with small size.

Achilles: A Tool for Contact Angle Estimation from Molecular Dynamics Simulations

In this work, a tool for estimating the contact angle from the molecular dynamics simulations is developed and presented. The tool (Achilles) can detect water droplet on hydrophobic and hydrophilic surfaces. The tool can reconstruct the droplets broken across the periodic boundaries. Further a neighbor density based accurate filter is used to find the droplet liquid vapor interface and a circle is fitted using it after removing the dense layers of water next to solid surface. This fitted circle is solved for contact angle and results are outputted in the form of graphical images and text. The entire content of the internal computations of the tool is broken down into 4 phases and users can monitor the outcomes at every phase through output images. The tool is tested using sample molecular dynamics results of water droplet on hydrophobic and hydrophilic surfaces. We believe this tool can be a good addition to the molecular dynamics simulation community who work on the interfacial physics, droplet evaporation, super hydrophobic surfaces, and wettability etc.

Computational Protocol for Spray Flow Simulations Including Primary Atomization

Primary atomization is the key element in spray flow simulations. We have, in our previous work, used and validated the integral form of the conservation equations, leading to the “quadratic formula” for determination of the drop size during spray atomization in various geometry. A computational protocol has been developed where this formulation is adapted to existing computational frameworks for continuous and dispersed (droplet) liquid phase, for simulations of pressure-atomized sprays with and without swirl. In principle, this protocol can be applied to any spray geometry, with appropriate modifications in the atomization criterion. The preatomization continuous liquid motion (e.g., liquid column or sheet) is computed using volume-of-fluid (VOF) or similar methods, then the velocity data from this computation is input to the quadratic formula for determination of the local drop size. This initial drop size, along with the local liquid velocities from VOF, is then used in a Lagrangian tracking algorithm for the postatomization dispersed droplet calculations. This protocol can be implemented on coarse-grid, time-averaged simulations of spray flows, and produces convincing results when compared with experimental data for pressure-atomized sprays with and without swirl. This approach is general, and can be adapted in any spray geometries for complete and efficient computations of spray flows.

Continuous 3D printing from one single droplet

3D printing has become one of the most promising methods to construct delicate 3D structures. However, precision and material utilization efficiency are limited. Here, we propose a one-droplet 3D printing strategy to fabricate controllable 3D structures from a single droplet ascribing to the receding property of the three-phase contact line (TCL) of the resin droplet. The well-controlled dewetting force of liquid resin on the cured structure results in the minimization of liquid residue and the high wet and net material utilization efficiency in forming a droplet into a 3D structure. Additionally, extra curing induced protruding or stepped sidewalls under high printing speed, which require high UV intensity, can be prevented. The critical is the free contact surface property of the droplet system with the introduction of the receding TCL, which increased the inner droplet liquid circulation and reduces the adhesion properties among the liquid resin, cured resin, and resin vat.

Using a Digital Microfluidic System to Evaluate the Stretch Length of a Droplet with a L-DEP and Varied Parameters

Digital microfluidics has become intensively explored as an effective method for liquid handling in lab-on-a-chip (LOC) systems. Liquid dielectrophoresis (L-DEP) has many advantages and exciting prospects in driving droplets. To fully realize the potential benefits of this technique, one must know the droplet volume accurately for its distribution and manipulation. Here we present an investigation of the tensile length of a droplet subjected to a L-DEP force with varied parameters to achieve precise control of the volume of a droplet. Liquid propylene carbonate served as a driving liquid in the L-DEP experiment. The chip was divided into two parts: an electrode of width fixed at 0.1 mm and a total width fixed at 1 mm. Each had a variation of six electrode spacings. The experimental results showed that the stretching length decreased with decreasing electrode width, but the stretching length did not vary with an increased spacing of the electrode. When the two electrodes were activated, the length decreased because of an increase in electrode spacing. The theory was based on the force balance on a droplet that involved the force generated by the electric field, friction force, and capillary force. The theory was improved according to the experimental results. To verify the theoretical improvement through the results, we designed a three-electrode chip for experiments. The results proved that the theory is consistent with the results of the experiments, so that the length of a droplet stretched with L-DEP and its volume can be calculated.

Experimental study on the effect of the liquid/surface thermal properties on droplet impact

Droplet impact on hot surfaces is widely encountered in industry and engineering applications. In the present paper we investigate the effect of the combination of the droplet liquid type and the solid surface type and their effect on droplet impact dynamics. We test three surfaces, copper 110, aluminum 1199 and stainless steel 304, and two liquids, water and ethanol. These surfaces and liquids are characterized by high and low thermophysical properties. The three surfaces are tested with water to investigate the effect of the surface on the droplet dynamics. After that, we test both liquids with aluminum. Our findings showed that the Leidenfrost temperature does not always correlate with the surface thermal properties as reported in the literature. Some surfaces can undergo changes because of the heating and this reduces their initial thermal properties. For this reason, such surfaces are capable to show two Leidenfrost temperatures because of the thermophysical properties reduction during heating. Our findings also revealed that the Leidenfrost temperature of liquids with low thermophysical properties including surface tension, evaporation latent heat and density show trivial effect by the droplet impact velocity; i.e. the Leidenfrost temperature show trivial increase by increasing the droplet impact velocity. Liquids with high thermophysical properties show significant Leidenfrost temperature increase by increasing the impacting velocity.

Droplet fluid infusion into a dust layer in relation to self-cleaning

Wettability of a droplet liquid on a dusty hydrophobic plate is considered and the fluid infusion into the dust layer is studied pertinent to dust removal from the hydrophobic surfaces via rolling/sliding droplets.