Spectacular Behavior of a Viscoelastic Droplet Impinging on a Superhydrophobic Mesh.

Spray formation using the droplet impact on superhydrophobic mesh surfaces is particularly important because of its application in different industries. The present study revealed that adding a trivial amount of the poly(ethylene oxide) (PEO) polymer to a water droplet can considerably change the impact phenomena on the superhydrophobic mesh surfaces and suppress the spray formation. Droplet rebound is observed only in a narrow range of impact velocities of PEO aqueous droplets when the tiny filaments still connect the surface and droplet. Rebound suppression and deposition of the PEO aqueous droplet is attributed to the higher interaction between the polymer chains and the superhydrophobic mesh surface. After a critical impact velocity and We number which is independent of the PEO concentration, the liquid penetrates the mesh pores. The penetrated liquid formed the ligaments that grow until they reach the maximum length and surprisingly retract back to the mesh surface and the mother droplet. The ligaments destabilized at low PEO concentrations (c = 0.5 and 1 g/L) and a mesh opening size of H = 357 µm to the crest swell droplets when the droplet size is reduced by increasing the impact velocity. The ligament fragmentation and droplet detachment are observed only at high impact velocities when c = 0.5 and 1 g/L and H = 357 µm. The result shows that the PEO additive does not significantly affect the maximum spreading diameter. An empirical model to calculate the maximum spreading factor is developed.

Successive Rebounds of Impinging Water Droplets on Superhydrophobic Surfaces.

When a water droplet strikes a superhydrophobic surface, there may be several to a few tens of rebounds before it comes to rest. Although this intriguing multiphase flow phenomenon has received a great deal of attention from interfacial scientists and engineers, the underlying dynamics have not yet been completely resolved. In this paper, we report on an experimental investigation into the bouncing behavior of water droplets impinging on macroscopically flat superhydrophobic surfaces. We show that the restitution coefficient, which quantifies the energy consumed during impact and rebound, exhibits a nonmonotonic dependence on the Weber number. It is the droplet-surface friction that restricts the rebound height of the impinging droplet, so its restitution coefficient increases with the Weber number when the impact velocity is below a critical value. Above this value, the viscous friction within a thin liquid layer close to the superhydrophobic surface becomes dominant, and thus, the restitution coefficient decreases sharply. On the basis of energy analyses, semiempirical formulas are proposed to describe the restitution coefficient, and these can be employed to predict the number of successive rebounds of impinging droplets on superhydrophobic surfaces.

Surface-Charge-Assisted Microdroplet Generation on a Superhydrophobic Surface.

The ability to generate and manipulate droplets down to microscales has attracted great attention in a variety of applications, such as in printing, microreactors, and biological assays. However, the production of microdroplets is often limited by special equipment or the size of needles. Here, an unexplored and facile approach is demonstrated; microdroplets can be generated and trapped yet not pinned on a micro-nano-structured superhydrophobic surface by controllable surface charge during drop impact. Tiny droplets with a size at a scale of tens of microns to millimeters are generated by simply changing the impacting velocity, the size of the impact drop, or impact frequency. Theoretical analysis suggests the generation of the microdroplet as a result of the surface-charge-regulated adhesion, competing with liquid dynamic and interfacial energy. The distribution of surface charge which determines the size and the location of the microdroplet is at the top of the micro-nano-structured surface and dependent on the pressure field applied on the surface during the drop impact. The mobility of the resulting microdroplet that can be easily manipulated without liquid retention is also shown, by taking advantage of the shielding property of the surface charge. This facile yet effective method provides a promising candidate for the realization of tiny droplet-generating and -manipulating applications.

Penetration and ligament formation of viscoelastic droplets impacting on the superhydrophobic mesh.

Spraying occurs by the impact of water droplets on the superhydrophobic wire meshes by liquid penetration during the spreading and recoiling. We have shown that adding a small amount of high molecular weight polymer (PEO) alters the ligaments formation and stabilizes them due to its high elasticity. Consequently, it suppresses droplet spray during droplet spreading and recoiling (recoil penetration). In the wide range of the impact velocities, the penetrated ligaments retracted back to the mesh after reaching the maximum length and eventually merged with the droplet on the mesh. The empirical fitting shows that the ligament evolution follows the parallel spring-dashpot model of Kelvin-Voigt. The additive polymer also changes the recoil penetration mechanisms from cavity collapse to cavity detachment due to the higher retraction velocity of the cavity near the mesh that is induced by the upward flow formed by the retraction of the ligaments to the mother droplet. A model based on mass conservation is proposed to calculate the variation of the maximum ligament size.

Janus Charged Droplet Manipulation Mediated by Invisible Charge Walls.

The ability to control the mobility and function of droplets is fundamental to developing open surface microfluidics. Despite notable progress in the manipulation of droplets, the existing strategies are still limited in functionalizing droplets. Herein, the coupling of droplet motion and functionalization elicited by an invisible charge wall is reported. The charged superamphiphobic surface is overlapped with a conductor to induce free charge, creating the invisible charge wall at the overlapping boundary. The charge wall can trap droplets and polarize them into Janus charged state. It is found that the trapping degree and the charge distribution in the Janus charged droplet depend on the original surface charge on the superamphiphobic surface. The invisible charge wall can also be established at diverse boundary curvatures, allowing to design pathways for droplet manipulations. Furthermore, the enrichment of protein and nanomaterial in the manipulated Janus charged droplet is demonstrated. The strategy provides a potential microfluidic platform with orthogonal functionalities.

In situ tunable droplet adhesion on a super-repellent surface via electrostatic induction effect.

In this paper, we report a finding that substrate affects the adhesion of charged super-repellent surfaces. Water droplet impacting on a super-repellent surface produces surface charge, whose expression depends on the substrate. The charged super-repellent surface is sticky to droplets for a suspended substrate made of dielectric materials, while it has low adhesion for a conducting substrate or stage attached at the bottom because of electrostatic induction. Theoretical analysis and simulation are conducted to elucidate the mechanism of substrate effect on surface adhesion. Finally, we develop a new approach to reversibly tune the adhesion of super-repellent surface by combining surface-charge-induced adhesion increase and electrostatic-induction-regulated express of net surface charge. As a proof-of-concept experiment, we demonstrate that droplet sorting and manipulations can be realized by using this controllable surface adhesion tuning approach, which has potential applications in advanced lab-on-a-drop platform.

Macrodrop-Impact-Mediated Fluid Microdispensing.

High-resolution fluid dispensing techniques play a critical role in modern digital microfluidics, micro-biosensing, and advanced fabrication. Though most of existing dispensers can achieve precise and high-throughput fluid dispensing, they suffer from some inherent problems, such as specially fabricated dispensing micronozzles/microtips, large operating systems, low volume tunability, and poor performance for low surface tension liquids and liquids containing solid/liquid additives. Herein, the authors propose a facile, low-frequency micro dispensing technique based on the Rayleigh-Plateau instability of singular liquid jets, which are stimulated by the air cavity collapse arising in the impact of microliter drops on non-wetting surfaces. This novel dispensing strategy is capable to produce single microdrops of low-viscosity liquids with a tunable volume from picoliters to nanoliters, and the operational surface tension range covers most laboratory solvents. The dispensing function is implemented without using small-dimension nozzles/tips and enables handling diverse complex liquids. Moreover, the rather simple operating platform allows the integration of the whole dispensing function into a handy portable device with a low cost. Employing this microdispensing technique, the authors have controlled microchemical reactions, handled liquid samples in biological analysis, and fabricated smart materials and devices. The authors envision that this rational microdrop generator would find applications in various research areas.

Facile Strategy to Generate Charged Droplets with Desired Polarities.

Water droplets are usually charged positively via either electrospray or contact electrification at the solid/liquid interface. Herein, we describe a facile two-step strategy to generate charged droplets with desired polarities. In particular, negatively charged droplets can be generated via electrostatic induction using a precharged superamphiphobic substrate as an electret. The interplay of repulsive and attractive interactions between like- and unlike-charged droplets or electret leads to rapid droplet transport and self-assembly of specific highly ordered arrays.

Prompting Splash Impact on Superamphiphobic Surfaces by Imposing a Viscous Part.

It is widely acknowledged that splash impact can be suppressed by increasing the viscosity of the impinging drop. In this work, however, by imposing a highly viscous drop to a low-viscosity drop, it is demonstrated that the splash of the low-viscosity part of this Janus drop on superamphiphobic surfaces can be significantly promoted. The underlying mechanism is that the viscous stress exerted by the low-viscosity component drives the viscous component moving in the opposite direction, enhancing the spreading of the low-viscosity side and thereby its rim instability. The threshold velocity, above which splashing occurs, can be tuned by varying the viscosity ratio of the Janus drop. Moreover, the impact of the Janus drop can be employed to verify the mechanism of splash.

Impact of viscous droplets on different wettable surfaces: Impact phenomena, the maximum spreading factor, spreading time and post-impact oscillation.

In this paper, we experimentally investigated the impact dynamics of different viscous droplets on solid surfaces with diverse wettabilities. We show that the outcome of an impinging droplet is dependent on the physical property of the droplet and the wettability of the surface. Whereas only deposition was observed on lyophilic surfaces, more impact phenomena were identified on lyophobic and superlyophobic surfaces. It was found that none of the existing theoretical models can well describe the maximum spreading factor, revealing the complexity of the droplet impact dynamics and suggesting that more factors need to be considered in the theory. By using the modified capillary-inertial time, which considers the effects of liquid viscosity and surface wettability on droplet spreading, a universal scaling law describing the spreading time was obtained. Finally, we analyzed the post-impact droplet oscillation with the theory for damped harmonic oscillators and interpreted the effects of liquid viscosity and surface wettability on the oscillation by simple scaling analyses.