Jetting Dynamics of Viscous Droplets on Superhydrophobic Surfaces.

We investigated the dynamics of liquid jets engendered by the impact of droplets on a fractal superhydrophobic surface. Depending on the impact conditions, jets emanate from the free liquid surface with several different shapes and velocities, sometimes accompanied by droplet ejection. Experimental outcomes exhibit two different regimes: the singular jet and columnar jet. We found that droplet impacts at a lower impact velocity and low viscosity result in singular jets, attaining a maximum velocity nearly 20-fold higher than the impact velocity. The high-speed video frames reveal that the formation and subsequent collapse of the cylindrical air cavities within the droplet favor the formation of these high-speed singular jets. In contrast, the capillary wave focusing engenders columnar jets at a moderate to high impact velocity. With an increase in viscosity, singular jets are suppressed at lower impact velocities, whereas columnar jets are seen regularly. The columnar jets ascend and grow over time, feeding a bulbous mass, and subsequently the bulb separates itself from the parent jet due to capillary pinch-off phenomena. The quantitative analysis shows that columnar jets' top jet drop size varies nonmonotonically and is influenced by preceding jetting dynamics. At moderate viscosity, the drop size varies with jet velocity, following a power-law scaling. At very high viscosities, both singular and columnar jetting events are inhibited. The results are relevant to several recent technologies, including microdispensing, thermal management, and disease transmission.

Magnetic-Field Mediated Active Propulsion of Ferrofluid Droplets on a Wire.

Droplet actuation on wires or fibers is highly relevant to digital microfluidics applications. Several different passive or active actuation mechanisms have been studied, including capillary pressure, thermal gradient, tilting, acoustics, and electrowetting, to name a few. However, the lack of precise control and accurate droplet positioning during its actuation on a wire limits the use of earlier methods. On the other hand, using a nonuniform magnetic field to actuate a droplet on a micrometer-size wire helps overcome the above limitations. In this context, we elucidate the motion of microliter-size ferrofluid droplets, smaller than the capillary length, in the presence of a permanent magnet using a simple yet robust experimental setup, wherein the clamshell shape droplets hang on a wire at a predetermined distance from the magnet. Beyond a critical volume, the droplet starts moving toward the magnet while exhibiting shape evolution continuously. Using a high-speed videography technique, we uncover the intricate relationship between the magnetowetting, favoring continuous shape evolution of droplets of different sizes, and their actuation dynamics (velocity) on a wire, which has scarcely been explored. The most significant contribution of this study is the detailed evaluation of the shape factor (k), relating the shape evolution of the droplet to its velocity through the contact angle hysteresis force. A theoretical framework has also been proposed, comprising different forces in action, which is found to set forth a good match with the experimental results. The results from this study are expected to open up new avenues in digital microfluidics, specifically in drug delivery, ferrobotics, and droplet logic gates, to name a few.

Magnetowetting dynamics of sessile ferrofluid droplets: a review.

The fascinating behavior of ferrofluids in a magnetic field has been intriguing researchers for many years. With the advancement in digital microfluidics, ferrofluid droplets have been extensively used in different applications ranging from biomedical to mechanical systems. Notably, the magnetic field can change the wetting dynamics of sessile ferrofluid droplets, leading to a plethora of interesting hydrodynamic phenomena. In the recent past, the spatiotemporal evolution of the droplet shape and contact line dynamics of a ferrofluid droplet in different magnetowetting scenarios has been explored widely. The relevant studies elucidate several critical aspects, such as the role of magnetic nanoparticles, carrier fluid, and the interaction of the magnetic fluid with the solid surface, among many others. Hence a systematic review of the progress made in understanding the fundamental and practical aspects of magnetowetting in the past decade (2010-2020) would be a helpful resource to the scientific community in the near future. Drawn by this motivation, an honest effort has been made in this Review to highlight the significant scientific findings concerning the sessile droplet magnetowetting phenomena within the timeline of interest. Several cutting-edge applications developed from the scientific findings in the purview of magnetowetting have also been discussed before outlining the conclusions and future areas of scope.

Magnetowetting dynamics of sessile ferrofluid drops on soft surfaces.

We investigate the magnetowetting behaviour of sessile ferrofluid droplets on elastomeric surfaces with different stiffness. The non-uniform magnetic field engenders deformation and splitting of the ferrofluid droplet, which is greatly influenced by the softness of the substrate. We observe that the decrease in the dynamic contact angle is maximum on the softest substrate, while the contact line remains pinned. Again, for an apparently rigid substrate, the contact radius decreases almost linearly, whereas the decrease in the contact angle appears to be lower than that of soft substrates. The contact line experiences a transition from "stick-slip" on rigid surfaces to "pinned" motion on soft surfaces, which is favoured by the formation of a large wetting ridge and smaller receding contact angle on the latter. We also find that the splitting time and splitting ratio (the ratio of the volume of the daughter droplet to that of the residual droplet) increase with an increase in the softness of the underlying substrate.

Experimental characterization of the growth dynamics during capillarity-driven droplet generation.

The transient dynamics of a growing droplet in a yarn is explored following the spatiotemporal evolution of the three-phase contact line as well as the liquid-air interface with the help of videographic techniques and subsequent image analyses. The spontaneous capillary flow of liquids in a porous network is used to generate a droplet on the freely hanging end of a yarn whose other end is dipped continuously in a liquid reservoir. The growing droplet initially moves upward due to surface tension until the attainment of a critical volume, beyond which gravity is able to pull it downward until detachment. Based upon the spatiotemporal trajectory of the three-phase contact line of the droplet, the entirety of the associated growth dynamics can be divided in three distinct regimes, namely, "radial growth," "axial growth," and "motion" stages. The transition from one to the other is governed by the subtle interplay between the capillary and the gravity forces. Several experimental fluids are considered to elucidate the effect of the fluid properties on the transient contact line and interfacial dynamics of drops. The kinetics of the three-phase contact line and the radius of the droplet is found to follow two distinct exponential scaling laws, developed through the combination of the relevant forces. A mathematical model has also been proposed to predict the critical volume of the growing droplet in relation to its final volume, beyond which gravity controls the transient dynamics.

Field induced anomalous spreading, oscillation, ejection, spinning, and breaking of oil droplets on a strongly slipping water surface.

Application of an electric field on an oil droplet floating on the surface of a deionized water bath showed interesting motions such as spreading, oscillation, and ejection. The electric field was generated by connecting a pointed platinum cathode at the top of the oil droplet and a copper anode coated with polymer at the bottom of the water layer. The experimental setup mimicked a conventional electrowetting setup with the exception that the oil was spread on a soft and deformable water isolator. While at relatively lower field intensities we observed spreading of the droplet, at intermediate field intensities the droplet oscillated around the platinum cathode, before ejecting out at a speed as high as ∼5 body lengths per second at even stronger field intensities. The experiments suggested that when the electric field was ramped up abruptly to a particular voltage, any of the spreading, oscillation, or ejection motions of the droplet could be engendered at lower, intermediate and higher field intensities, respectively. However, when the field was ramped up progressively by increasing by a definite amount of voltage per unit time, all three aforementioned motions could be generated simultaneously with the increase in the field intensity. Interestingly, when the aforementioned setup was placed on a magnet, the droplet showed a rotational motion under the influence of the Lorentz force, which was generated because of the coupling of the weak leakage current with the externally applied magnetic field. The spreading, oscillation, ejection, and rotation of the droplet were found to be functions of the oil-water interfacial tension, viscosity, and size of the oil droplet. We developed simple theoretical models to explain the experimental results obtained. Importantly, rotating at a higher speed broke the droplet into a number of smaller ones, owing to the combined influence of the spreading due to the centripetal force and the shear at the oil-water interface. While the oscillatory and rotational motions of the incompressible droplet could be employed as stirrers or impellers inside microfluidic devices for mixing applications, the droplet ejection could be employed for futuristic applications such as payload transport or drug delivery.