Interface Dynamics at a Four-Fluid Interface during Droplet Impact on a Two-Fluid System.

We investigate the interfacial dynamics involved in the impact of a droplet on a liquid-liquid system, which involves the impingement of an immiscible core liquid drop from a vertical separation onto an interfacial shell liquid layer floating on a host liquid bath. The dynamics have been studied for a wide range of impact Weber numbers and two different interfacial shell liquids of varying volumes. The core drop, upon impact, dragged the interfacial liquid into the host liquid, forming an interfacial liquid column with an air cavity inside the host liquid bath. The dynamics is resolved into cavity expansion and rapid contraction, followed by thinning of the interfacial liquid. The interplay of viscous dissipation, interfacial pull, and core drop inertia influenced the necking dynamics. The viscous dissipation increases with the thickness of the interfacial layer, which depends on the volume and lateral spread over the water. The necking dynamics transitioned from inertia-dominated deep seal closure at higher spread, lower interfacial film volumes, and higher Weber numbers into inertia-capillary-dominated deep seal closure with an increase in film volumes, decrease in the spread of the interfacial fluid, or decrease in Weber number and finally into a no-seal closure at high volumes, low spread, and low Weber numbers.

Directional Manipulation of Drops and Solids on a Magneto-Responsive Slippery Surface.

The cloaking of the droplet and solid spheres by a thin ferrofluid layer forms a ferrofluid-wetting ridge, enabling the magnet-assisted directional manipulation of droplets and solid spheres on the magneto-responsive slippery surface. Understanding the interplay of various forces governing motion unravels the manipulation mechanism. The transportation characteristics of droplets and solid spheres on such surfaces enable their controlled manipulation in multiple applications. Here, we prepare magneto-responsive slippery surfaces by using superhydrophobic coatings on glass slides, creating a porous network and impregnating them with ferrofluid. Using a permanent magnet (and its translation) in the proximity of these surfaces, we manipulate the motion of liquid drops and solid spheres. Upon dispensing the droplet on the magneto-responsive slippery surface, the droplet is cloaked by a thin ferrofluid layer and forms a ferrofluid wetting ridge. Incorporating the magnetic field creates a magnetic-nanoparticle-rich zone surrounding the ferrofluid ridge. Thereafter, the motion of the magnet gives rise to the movement of the droplet. We found that the interplay of the magnetic force and viscous drag leads to the magnetic manipulation of droplets in a controlled fashion up to a certain magnet speed. Increasing the magnet speed further results in the uncontrolled motion of the droplet, where the droplet cannot follow the magnet trajectory. Moreover, we delineate multifunctional droplet manipulations, such as trapping, pendant droplet manipulation, coalescence, and microchemical reactions, which have wide engineering applications.

Density Functional Theory Approach to Interpret Elastowetting of Hydrogels.

Sessile hydrogel drops on rigid surfaces exhibit a wetting/contact morphology intermediate between liquid drops and glass spheres. Using density functional theory, we reveal the contact forces acting between a hydrogel and a rigid glass surface. We show that while transitioning from liquid-like to solid-like hydrogels, there exists a critical hydrogel elasticity that enables a switch from attractive-to-repulsive interaction with the underlying rigid glass surface. Our theoretical model is validated by experimental observations of sessile polyacrylamide hydrogels of varying elasticity on glass surfaces. Further, the proposed model successfully approaches Young's law in the pure liquid limit and work of adhesion in the glassy limit. Lastly, we show a modified contact angle relation, taking into account the hydrogel elasticity to explain the features of a distinct hydrogel foot.

Bacteria-surface interactions: role of impacting bacteria-laden droplets.

Understanding the interactions of pathogenic droplets with surfaces is crucial to biomedical applications. In this study, using E. coli as the model microbe, we investigate the impact of a bacteria-laden droplet on different substrates, both bare and antimicrobial. In doing so, we unveil the significance of kinetic energy and spreading parameters of the impacting droplet in determining the microbes' proliferation capabilities. Our results indicate an inverse relationship between the impact Weber number and the bacterial ability to proliferate. We reveal that the mechanical stress generated during impact impedes the capabilities of microbes present inside the droplet to create their progeny. Following an order analysis of the mechanical stress generated, we argue that the impact does not induce lysis-driven cell death of the bacteria; rather, it promotes a stress-driven transition of viable bacteria to a viable-but-non-culturable (VBNC) state. Furthermore, variations in the concentration of particles on the antimicrobial surfaces revealed the role of the post-impact spreading behaviour in dictating bacterial proliferation capabilities. Contrary to the conventional notion, we demonstrate that during the early stages of interaction, a bare substrate may outperform an antibacterial substrate in the inactivation of the bacterial load. Finally, we present an interaction map illustrating the complex relationship between bacterial colony-forming units, bactericide concentration on the surface, and the impact Weber number. We believe that the inferences of the study, highlighting the effect of mechanical stresses on the soft cell wall of microbes, could be a useful design consideration for the development of antimicrobial surfaces.

Flexible hydrogels connecting adhesion and wetting.

Raindrops falling on window-panes spread upon contact, whereas hail can cause dents or scratches on the same glass window upon contact. While the former phenomenon resembles classical wetting, the latter is dictated by contact and adhesion theories. The classical Young-Dupre law applies to the wetting of pure liquids on rigid solids, whereas conventional contact mechanics theories account for rigid-on-soft or soft-on-rigid contacts with small deformations in the elastic limit. However, the crossover between adhesion and wetting is yet to be fully resolved. The key lies in the study of soft-on-soft interactions with material properties intermediate between liquids and solids. In this work, we translate adhesion to wetting by experimentally probing the static signature of hydrogels in contact with soft PDMS of varying elasticity of both the components. Consequently, we probe this transition across six orders of magnitude in terms of the characteristic elasto-adhesive parameter of the system. In doing so, we reveal previously unknown phenomenology and a theoretical model which smoothly bridges adhesion of glass spheres with total wetting of pure liquids on any given substrate.

Cs+ conductance in graphene membranes with Ångström-scale pores: the role of pore entrance geometry.

Recently, 2D materials, such as graphene, have been successfully implemented as artificial conduits of molecular sizes. The extreme precision with which these structures can be fabricated provides an unprecedented framework for the development of highly specific and efficient devices. In this work, we study the electrophoretic transport of Cs+ ions in a graphene membrane with effective pore heights of 3.4 Å by conducting molecular dynamics simulations. The entrance of the pore is systematically modified to investigate the effect of pore geometry on ionic conductance. Simulation results suggest a significant correlation between ionic conductance and entrance geometry, with a variation of the conductance up to 100% across the studied cases. To explain the observed correlation, two mechanisms involving an intimate relationship between ion dehydration and edge functional groups are proposed. The present study provides theoretical insights that can aid the design of graphene-based membranes with tunable ionic transport properties.

Experimental observation and characterization of amorphous carbon generated in graphene on gold nanoparticles.

The interaction of graphene with gold nanoparticles is investigated using transmission electron microscopy. We observe gold-nanoparticle-mediated etching of graphene flakes, often leading to hole formation. Further, using a combination of high-angle annular dark field imaging and electron energy loss spectroscopy, we highlight that the catalytic effects of gold nanoparticles on graphene lead to the formation of amorphous carbon layers. From the extracted diffractograms, we observe regions with diffraction halos as well as some regions with a weak tetrahedral motif. Using independently performed Raman measurements, we confirm the presence of tetrahedral amorphous carbon as well as mixed graphitic-amorphous regions. For the amorphous carbon regions with mixed sp2-sp3 states, the Raman G peak is red-shifted to 1564 cm-1 and an I D/I G ratio of 0.63 indicates less than 20% sp3 content. For the tetrahedral amorphous carbon regions, we observe that the Raman G peak is at 1580 cm-1, close to that of monolayer graphene. However, there is no Raman D peak, i.e., I D/I G = 0, which indicates close to 100% sp3 content. The translation of the Raman G peak location and the I D/I G ratios is on par with the amorphization trajectory analysis of Ferrari and Robertson (Phys. Rev. B: Condens. Matter Mater. Phys., 2000, 61, 14095) and validates the conversion route of graphite to amorphous carbon to tetrahedral amorphous carbon. The presented method provides a promising pathway for creating defect-induced amorphous carbon at room temperature, which has a broader impact on the electronics and semiconductor industries.

Unravelling soft interfaces: Visualization of gel ridges.

HYPOTHESIS: Soft materials, particularly elastomers, are extensively studied, but investigations into purely soft gel contact systems are limited due to their complex dual phases consisting of polymer and free liquids. While Dual Wavelength-Reflection Interference Confocal Microscopy (DW-RICM) is effective for noninvasively visualizing interfaces from a bottom view, it faces challenges in gel studies due to close refractive indices of polymeric networks and free liquids. We hypothesize that modulating the refractive index of soft gels using nanoparticles (NPs) enhances the visualization of contact zone beneath the free surface, providing insights into the configuration of phase-separated free oil within gel-on-gel contact systems. EXPERIMENTS: Gel-on-gel contact systems were fabricated using immiscible organogels and hydrogels. Titanium dioxide (TiO2) NPs were introduced into the organogel to modulate refractive indices. Given the lack of prior studies on the hidden contact zone between gels, various techniques, including DW-RICM, side-view imaging, and inverted optical microscopy, were employed to observe and validate our findings. Comparative analyses were conducted with elastomer-on-rigid, elastomer-on-gel, and gel-on-rigid contact systems. FINDINGS: Our investigation demonstrated that a minimal amount of TiO2 NPs effectively delineates the direct contact radius between organogel polymeric networks and hydrogel surfaces. Comparative experiments showed that TiO2 addition did not alter the gels' mechanical and surface properties but significantly enhanced information on gel contact deformation. This enhanced visualization technique has the potential to advance our understanding of adhesive contacts in gels, providing valuable insights into interface phenomena involving biological soft tissues and cells.

New insights and novel perspectives in bileaflet mechanical heart valve prostheses thromboresistance.

BACKGROUND: Although well-known for their thromboresistance, bileaflet mechanical heart valves (BMHV) require lifelong anti-thrombotic therapy. This must be associated with a certain level of thrombogenicity. Since both thromboresistance and thrombogenicity are explained by the blood-artificial surface or liquid-solid interactions, the aim of the present study was to explore BMHV thromboresistance from new perspectives. The wettability of BMHV pyrolytic carbon (PyC) occluders was investigated in under-liquid conditions. The submerged BMHV wettability clarifies the mechanisms involved in the thromboresistance. METHODS: The PyC occluders of a SJM Regent™ BMHV were previously laser irradiated, to create a surface hierarchical nano-texture, featuring three nano-configurations. Additionally, four PyC occluders of standard BMHV (Carbomedics, SJM Regent™, Bicarbon™, On-X®), were investigated. All occluders were evaluated in under-liquid configuration, with silicon oil used as the working droplet, while water, simulating blood, was used as the surrounding liquid. The under-liquid droplet-substrate wetting interactions were analyzed using contact angle goniometry. RESULTS: All the standard occluders showed very low contact angle, reflecting a pronounced affinity for non-polar molecules. No receding of the contact line could be observed for the untreated occluders. The smallest static contact angle of around 61° could be observed for On-X® valve (the only valve made of full PyC). The laser-treated occluders strongly repelled oil in underwater conditions. A drastic change in their wetting behaviour was observed depending on the surrounding fluid, displaying a hydrophobic behaviour in the presence of air (as the surrounding medium), and showing instead a hydrophilic nature, when surrounded by water. CONCLUSIONS: BMHV "fear" water and blood. The intrinsic affinity of BMHV for nonpolar fluids can be translated into a tendency to repel polar fluids, such as water and blood. The blood-artificial surface interaction in BMHV is minimized. The contact between blood and BMHV surface is drastically reduced by polar-nonpolar Van der Waals forces. The "hydro/bloodphobia" of BMHV is intrinsically related to their chemical composition and their surface energy, thus their material: PyC indeed. Pertaining to thromboresistance, the surface roughness does not play a significant role. Instead, the thromboresistance of BMHV lies in molecular interactions. BMHV wettability can be tuned by altering the surface interface, by means of nanotechnology.