The impact of droplets on textured or rough surfaces has garnered remarkable appreciation due to its multifarious applications such as self-cleaning, anti-icing, and anti-fouling, leading to a plethora of engineered superhydrophobic surfaces (SHPs) exhibiting different interfacial dynamics during impact. However, the prime limiting factors in using these surfaces abundantly arise from their long fabrication time and concurrent high cost. Here, we propose using carbon soot nanoparticle (CSNPs) coated fractal superhydrophobic surfaces prepared from flame deposition as an alternative to overcome the limitations. We establish our claim by exploring the dynamic wetting behavior of the soot-coated surface in terms of key droplet impact parameters such as rebounding, contact time, impalement transition, and splashing dynamics. A systematic investigation is undertaken by considering a vast range of viscosity and impact conditions. One of the significant observations is the absence of the partial rebound regime during the impact of water droplets on the CSNPs surface, unlike most of the existing superhydrophobic surfaces under similar impact conditions. Furthermore, the surface promotes droplet splashing for moderately viscous solutions at high impact velocities, also characterized by unified scaling laws based on different non-dimensional numbers. Finally, a regime map is proposed to elucidate the complete dynamic wetting characteristics of these CSNPs' surfaces for viscous fluids, which further reflects competitive and equal, if not superior, wetting behavior compared to a series of existing non-wetting surfaces. The results are expected to promote CSNPs based surfaces in applications such as self-cleaning, oil-water separation, and thermal management.
This article presents a systematic review of the progress made in understanding the fundamental and practical aspects of sessile droplet magnetowetting phenomena in the past decade (2010–2020).
The main analytical model for describing the motion of magnetic domain walls is the 1-D model formulated based on the profile of a Bloch wall. This model qualitatively describes the motion of magnetic domain wall in nanowires, while it may fail to match experimental and numerical results quantitatively. In recent years, the 1-D model has been further generalized by the introduction of terms such as spin transfer torques and spin orbit torques. It has also been used to describe the motion of different domain walls, including vortex walls. It seems that in many such attempts, formalisms are not followed accurately and the main assumptions of the model (such as the Bloch wall profile used in developing the model) are underestimated. In this paper, we first derive an analytical model to describe the motion of a tilting Bloch wall in perpendicularly magnetized materials using four collective coordinates. We then compare the energy landscape predicted by this model to that of micromagnetic simulations, highlighting the possibility of using such comparisons to develop corrections for the 1-D model
We study bubble domain wall dynamics using micromagnetic simulations in perpendicularly magnetized ultra-thin films with disorder and Dzyaloshinskii-Moriya interaction. Disorder is incorporated into the material as grains with randomly distributed sizes and varying exchange constant at the edges. As expected, magnetic bubbles expand asymmetrically along the axis of the in-plane field under the simultaneous application of out-of-plane and in-plane fields. Remarkably, the shape of the bubble has a ripple-like part which causes a kink-like (steep decrease) feature in the velocity versus in-plane field curve. We show that these ripples originate due to the nucleation and interaction of vertical Bloch lines. Furthermore, we show that the Dzyaloshinskii-Moriya interaction field is not constant but rather depends on the in-plane field. We also extend the collective coordinate model for domain wall motion to a magnetic bubble and compare it with the results of micromagnetic simulations.
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.
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