Dynamics of droplet impingement on bioinspired surface: insights into spreading, anomalous stickiness and break-up

Surface Instability in a Nematic Elastomer

Liquid crystal elastomers (LCEs) are soft phase-changing solids that exhibit large reversible contractions upon heating, Goldstone-like soft modes, and resultant microstructural instabilities. We heat a planar LCE slab to isotropic, clamp the lower surface, then cool back to nematic. Clamping prevents macroscopic elongation, producing compression and microstructure. We see that the free surface destabilizes, adopting topography with amplitude and wavelength similar to thickness. To understand the instability, we numerically compute the microstructural relaxation of a "nonideal" LCE energy. Linear stability reveals a Biot-like scale-free instability, but with oblique wave vector. However, simulation and experiment show that, unlike classic elastic creasing, instability culminates in a crosshatch without cusps or hysteresis, and is constructed entirely from low-stress soft modes.

Damped Oscillatory Dynamics of a Drop Impacting over Oil-Infused Slippery Interfaces─Does the Oil Viscosity Slow it Down?

A liquid drop impacting on a soft surface is known to exhibit fascinating dynamics that is distinctive from its bounce-back atop a rigid surface. However, while the early spreading of the drop subsequent to its immediate impact with a lubricating liquid layer appears to be reasonably well understood, the later events of retraction and eventual stabilization appear to be poorly addressed. Here, we bring out the nontrivial confluence of the solid substrate wettability and the liquid layer viscosity toward modulating the post-collision dynamics of an impinging liquid drop on a viscous oil-infused surface during its later phase of settlement before arriving at an equilibrium state. Our results reveal that despite an intuitive analogy with the classical phenomenon of damped oscillation, the drop, during its later stages of motion, undergoes dynamical events that may be nontrivially dictated by not only the relative viscosity of the impacting drop and the liquid layer but also the intrinsic wettability of the solid substrate, governing its post-impact settlement via a sequel of spreading-retraction cycles. As a consequence, the viscous liquid layer, instead of providing additional damping, may nonintuitively reduce the effective viscous dissipation so as to hasten the drop's final settlement. These results may turn out to be critical in designing engineered surfaces for tuning the movement of drops in a preferential pathway, bearing decisive implications in the functionalities of liquid lenses, inkjet printing, spray coating and cooling, and several other emerging applications in the realm of lubricated fluidic interfaces.

Droplet Impact Dynamics on Biomimetic Replica of Yellow Rose Petals: Rebound to Micropinning Transition

Rose petals exhibit a phenomenal wetting property of being sticky and superhydrophobic simultaneously. A recent study has shown that for short timescales, associated with drop impact phenomenon, lotus leaf and rose petal replicas exhibit similar wettability, thereby highlighting the difference between long and short time wettability. Also, short time wetting on rose petals of different colors remains completely unaddressed, as almost all existing study on wetting of rose petals have been performed with the classical red rose (Rosa chinensis). In this paper, we compare the drop impact studies on replicas of a yellow rose petal, with those on extensively studied red rose petal replicas and the lotus leaf over a wide range of Weber number (We), by varying the height of fall (h) from 10 to 375 mm. Our results reveal that over the replica of a yellow rose petal, the initial impact outcome varies from complete rebound to micro pinning and eventually complete pinning depending on the kinetic energy of the impacting drop, in contrast to that on red rose petal replica on which the droplet always pinned. Based on experimental finding, we present a comprehensive regime phase map of the post impact behavior of the drop on different surfaces as a function of impact height. We also present a simple scaling analysis to understand the combined effect of pattern height and periodicity on the critical h corresponding to wetting regime transition. Additionally, variation of maximum spreading diameter and spreading time with the h for the different surfaces is also discussed. The results highlight that the initial impact dynamics of a water drop over a topographically patterned substrate is a strong function of the topographical parameters and can be very different from the equilibrium wetting state.

Nanometer-Thick Superhydrophobic Coating Renders Cloth Mask Potentially Effective against Aerosol-Driven Infections

The advent of COVID-19 pandemic has made it necessary to wear masks across populations. While the N95 mask offers great performance against airborne infections, its multilayered sealed design makes it difficult to breathe for a longer duration of use. The option of using highly breathable cloth or silk masks especially for a large populace is fraught with the danger of infection. As a normal cloth or silk mask absorbs airborne liquid, it can be a source of plausible infection. We demonstrate the chemical modification of one such mask, Eri silk, to make it hydrophobic (contact angle of water is 143.7°), which reduces the liquid absorption capacity without reducing the breathability of the mask significantly. The breathability reduces only 22% for hydrophobic Eri silk compared to the pristine Eri silk, whereas N95 shows a 59% reduction of breathability. The modified hydrophobic silk can repel the incoming aqueous liquid droplets without wetting the surface. The results indicate that a multilayered modified silk mask to make it hydrophobic can be an affordable and breathable alternative to the N95 mask.

Improved Mesenchymal Stem Cell Proliferation, Differentiation, Epithelial Transition, and Restrained Senescence on Hierarchically Patterned Porous Honey Silk Fibroin Scaffolds

We report a significant improvement of adipose-derived mesenchymal stem cells' (ADMSCs) biocompatibility and proliferation on hierarchically patterned porous honey-incorporated silk fibroin scaffolds fabricated using a combination of soft lithography and freeze-drying techniques. Parametric variations show enhanced surface roughness, swelling, and degradation rate with good pore interconnectivity, porosity, and mechanical strength for soft-lithographically fabricated biomimetic microdome arrays on the 2% honey silk fibroin scaffold (PHSF2) as compared to its other variants, which eventually made PHSF2 more comparable to the native environment required for stem cell adhesion and proliferation. PHSF2 also exhibits sustained honey release with remarkable antibacterial efficacy against methicillin-resistant Staphylococcus aureus (MRSA). Honey incorporation (biochemical cue) influences microdome structural features, that is, biophysical cues (height, width, and periodicity), which further allows ADMSCs pseudopods (filopodia) to grasp the microdomes for efficient cell-cell communication and cell-matrix interaction and regulates ADMSCs behavior by altering their cytoskeletal rearrangement and thereby increases the cellular spreading area and cell sheet formation. The synergistic effect of biochemical (honey) and biophysical (patterns) cues on ADMSCs studied by the nitro blue tetrazolium assay and DCFDA fluorescence spectroscopy reveals limited free radical generation within cells. Molecular expression studies show a decrease in p53 and p21 expressions validating ADMSCs senescence inhibition, which is further correlated with a decrease in cellular senescence-associated β galactosidase activity. We also show that an increase in CDH1 and CK19 molecular expressions along with an increase in SOX9, RUNX2, and PPARγ molecular expressions supported by PHSF2 justify the substrate's efficacy of underpinning mesenchymal to epithelial transition and multilineage trans-differentiation. This work highlights the fabrication of a naturally healing nutraceutical (honey)-embedded patterned porous stand-alone tool with the potential to be used as smart stem cells delivering regenerative healing implant.

Flow and deformation characteristics of a flexible microfluidic channel with axial gradients in wall elasticity

Axial gradients in wall elasticity may have significant implications in the deformation and flow characteristics of a narrow fluidic conduit, bearing far-reaching consequences in physiology and bio-engineering. Here, we present a theoretical and experimental framework for fluid-structure interactions in microfluidic channels with axial gradients in wall elasticity, in an effort to arrive at a potential conceptual foundation for in vitro study of mirovascular physiology. Towards this, we bring out the static deformation and steady flow characteristics of a circular microchannel made of polydimethylsiloxane (PDMS) bulk, considering imposed gradients in the substrate elasticity. In particular, we study two kinds of elasticity variations - a uniformly soft (or hard) channel with a central strip that is hard (or soft), and, increasing elasticity along the length of the channel. The former kind yields a centrally constricted (or expanded) deformed profile in response to the flow. The latter kind leads to increasingly bulged channel radius from inlet to outlet in response to flow. We also formulate an analytical model capturing the essential physics of the underlying elastohydrodynamic interactions. The theoretical predictions match favourably with the experimental observations and are also in line with reported results on stenosis in mice. The present framework, thus, holds the potential for acting as a fundamental design basis towards developing in vitro models for micro-circulation, capable of capturing exclusive artefacts of healthy and diseased conditions.

Durability of submerged hydrophobic surfaces

Hydrophobic and superhydrophobic surfaces have gained wide popularity due to their potential in various areas such as in self-cleaning and anti-fouling materials, drag reduction and microfluidics. However, for all practical applications, the long term durability of these surfaces is extremely important, yet not often investigated. Of particular interest is the long term durability of soft hydrophobic surfaces that remain submerged underwater for a prolonged duration. In this article, we explore how the chemical durability of flat and patterned crosslinked PDMS surfaces (polydimethylsiloxane, a preferred material for microfabrication) change as a function of time when submerged in acidic, basic and neutral media for different durations over a prolonged period of time. Based on contact angle measurements, atomic force microscopy, confocal microscopy and SEM analysis of the surfaces, we checked if there is any change in the morphology of the surface due to deposition or etching. We created a biomimetic positive replica of a lotus leaf that exhibited super-hydrophobicity and Cassie state of wetting with a static water contact angle (θ) > 150°, and compared the degradation with a negative replica of lotus leaf (θ ∼ 127°), a grating patterned surface that exhibited Wenzel state of wetting (θ ∼ 110°) and a flat crosslinked PDMS surface (θ ∼ 105°). The positive replica maintained reasonable hydrophobicity (θ > 90°) for up to a month, but lost its super-hydrophobic property. The surface hydrophobicity degraded the most in the case of basic solution due to deposition.