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

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.

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.

Axial Alignment of Carbon Nanotubes on Fibers To Enable Highly Conductive Fabrics for Electromagnetic Interference Shielding

Conductive coatings show great promise for next-generation electromagnetic interference (EMI) shielding challenges on textile; however, their stringent requirements for electrical conductivity are difficult to meet by conventional approaches of increasing the loading and homogeneity of conductive nanofillers. Here, the axial alignment of carbon nanotubes (CNTs) on fibers that were obtained by spontaneous capillary-driven self-assembly is shown on commercial cotton fabrics, and its great potential for EMI shielding is demonstrated. The aligned CNTs structurally optimize the conductive network on fabrics and yield an 81-fold increase in electrical conductivity per unit of CNT, compared with the disordered CNT microstructure. The high-efficiency electrical conductivity allows a several-micron-thick coating on insulating fabrics to endow an EMI shielding effectiveness of 21.5 dB in the X band and 20.8 dB in the Ku band, which meets the standard shielding requirement in commercial applications. It is among the minimum reported thicknesses for conductive nanocomposite coatings to date. Moreover, the coated fabrics with aligned CNTs possess a desirable stability upon bending, scratching, stripping, and even washing, which is attributed to the dense CNT packing in the aligned microarchitecture. This work presents the anisotropic structure on large areas by self-assembly, offering new opportunities for next-generation portable and wearable electronic devices.