Electrothermally Assisted Surface Charge Density Gradient Printing to Drive Droplet Transport

Abstract

Surface 2019, surface charge density (SCD) gradient printing-driven droplet transport, has attracted considerable attention as a novel and effective approach, which adopts the water droplet impacting a nonwetting surface to create a reprintable SCD gradient pathway conveniently and realizes the high-velocity and long-distance transport of droplets. In the present work, we further investigated the effects of electrothermal behavior on SCD gradient printing on hydrophobic surfaces by considering the droplet impact dynamics. After the electrothermal function was activated, the wettability of the hydrophobic surface improved in terms of the spreading factor history and the infiltration depth, which increased the probability of solid/liquid contact electrification to generate a more favorable SCD gradient. Since the hydrophobic surface was negatively charged by droplet impact, polarized droplets rolled forward along the preprinted SCD gradient pathway due to opposite charge attraction. Based on these results, we designed a SCD gradient printer with an electrothermal function for hydrophobic surfaces. Subsequently, the kinematic parameters of rolling droplets on hydrophobic surfaces were observed and quantified to evaluate the improvements resulting from the electrothermal function.

Surface Charge Density Gradient Printing To Drive Droplet Transport: A Numerical Study

“…Furthermore, considering that the SCD is easily affected by environmental humidity, the authors further investigated experimentally the roles of electrothermal behavior on the SCD gradient printing strategy. 34 We found that after activating the electrothermal function, the infiltrated depth of a falling droplet on the superhydrophobic surface was increased to generate a larger solid−liquid contact area. A larger contact area raised the probability of solid−liquid contact electrification and produced a more favorable SCD on the superhydrophobic surface.…”

“…To study the dynamic transport behavior of water droplets along a superhydrophobic surface with the preprinted SCD gradient, the Navier−Stokes equation should be solved numerically by coupling the electric force, which is different from our previous work. 34 The liquid−gas interface also needs to be captured using the computational fluid dynamics (CFD) tool.…”

“…It is worth mentioning that the superhydrophobic surface here was prepared using a spraying technology, the details of which can be found in our previous work. 34 Finally, by setting up a space−time coordinate system in the software, Tracker, the relevant kinematic parameters of the centroid point of the moving droplet were extracted from the high-speed videos. The above transport experiment of a single water droplet along the SCD gradient pathway was carried out in parallel three times, and then, the average kinematic data were plotted.…”

“…For the gas domain Ω g , the left and right boundary conditions were applied to the inlet ∂Ω g in and outlet ∂Ω g out of gas, respectively, and the boundary condition at the top of the gas domain Ω g was open to bulk gas ∂Ω g bulk with zero pressure. In the settings of surface domain Ω s as a wetted wall, a contact angle θ was specified to be 142.4° according to the experimental measurement . Meanwhile, the Navier slip boundary condition was adopted for the current wetted wall, which included a frictional force, bold-italicf bold-italicf = μ β normals bold-italicv normals where v s is the slip velocity, and β s is the slip length and was set to 0.5 h min , where h min is the size of the smallest element in the component (corresponding to the elemental size in the wall normal direction for elements of the boundary layer).…”

“…Moreover, the SCD gradient on the transport pathway can be easily erased by an ionizing blower and subsequently reprinted to realize excellent reprogrammability, which is almost impossible to the chemical or morphological gradient strategy. Furthermore, considering that the SCD is easily affected by environmental humidity, the authors further investigated experimentally the roles of electrothermal behavior on the SCD gradient printing strategy . We found that after activating the electrothermal function, the infiltrated depth of a falling droplet on the superhydrophobic surface was increased to generate a larger solid–liquid contact area.…”