Sepideh Khodaparast

Bubble-Driven Detachment of Bacteria from Confined Microgeometries

Moving air-liquid interfaces, for example, bubbles, play a significant role in the detachment and transport of colloids and microorganisms in confined systems as well as unsaturated porous media. Moreover, they can effectively prevent and/or postpone the development of mature biofilms on surfaces that are colonized by bacteria. Here we demonstrate the dynamics and quantify the effectiveness of this bubble-driven detachment process for the bacterial strain Staphylococcus aureus. We investigate the effects of interface velocity and geometrical factors through microfluidic experiments that mimic some of the confinement features of pore-scale geometries. Depending on the bubble velocity U, at least three different flow regimes are found. These operating flow regimes not only affect the efficiency of the detachment process but also modify the final distribution of the bacteria on the surface. We organize our results according to the capillary number, [Formula: see text], where μ and γ are the viscosity and the surface tension, respectively. Bubbles at very low velocities, corresponding to capillary numbers Ca < 5 × 10-5, exhibit detachment efficiencies of up to 80% at the early stage of bacterial adhesion. In contrast, faster bubbles at capillary numbers Ca > 10-3, have lower detachment efficiencies and cause significant nonuniformities in the final distribution of the cells on the substrate. This effect is associated with the formation of a thin liquid film around the bubble at higher Ca. In general, at higher bubble velocities bacterial cells in the corners of the geometry are less influenced by the bubble passage compared to the central region.

Dewetting of Thin Liquid Films Surrounding Air Bubbles in Microchannels

As an air bubble translates in a microchannel, a thin film of liquid is formed on the bounding walls. In a microchannel with a rectangular cross-section, the liquid in the film leaks toward the low-pressure corners of the geometry, which leads to the appearance of local minima in the film thickness in the cross-sectional plane. In such a configuration, theory suggests that the minimum film thickness scales with Ca and Ca4/3 depending on the distance from the nose of the bubble, where Ca = μUb/γ is the flow capillary number based on the bubble velocity Ub, liquid viscosity μ, and surface tension γ, and Ca ≪ 1. We show that the film of a partially wetting liquid dewets on the channel wall at the sites of the local minima in the film thickness as it acquires thicknesses around and below 100 nm. Our experiments show that the distance Lw between the nose of the bubble and the initial dewetting location is a function of Ca and surface wettability. For channels of different wettability, Lw always scales proportional to Caα, where 1.7 < α < 2 for the range of 10-5 < Ca < 10-2. Moreover, Lw increases up to 10 times by enhancing the wettability of the surface at a given Ca. Our present measurements of Lw provide a design constraint on the lengths of bubbles to maintain a liquid wet channel without dry patches on the wall.

Protocol to perform pressurized blister tests on thin elastic films

Abstract.This work aims to identify common challenges in the preparation of the blister test devices designed for the measurement of the energy release rate for brittle thin films and to propose easy-to-implement solutions accordingly. To this end, we provide a step-by-step guide for fabricating a blister test device comprised of thin polystyrene films adhered to glass substrates. Thin films are first transferred from donor substrates to an air-water interface, which is then used as a platform to locate them on a receiver substrate. We embed a microchannel at the back of the device to evacuate the air trapped in the opening, through which the pressure is applied. We quantify the height and the radius of the blister to estimate the adhesion energy using the available expressions correlating the normal force and the moment with the shape of the blister. The present blister test provided an adhesion energy per unit area of G = 18±2 mJ/m^2 for polystyrene on glass, which is in good agreement with the measurement of G = 14±2 mJ/m^2 found in our independent cleavage test.Graphical abstract

Water-Based Peeling of Thin Hydrophobic Films

Inks of permanent markers and waterproof cosmetics create elastic thin films upon application on a surface. Such adhesive materials are deliberately designed to exhibit water-repellent behavior. Therefore, patterns made up of these inks become resistant to moisture and cannot be cleaned by water after drying. However, we show that sufficiently slow dipping of such elastic films, which are adhered to a substrate, into a bath of pure water allows for complete removal of the hydrophobic coatings. Upon dipping, the air-water interface in the bath forms a contact line on the substrate, which exerts a capillary-induced peeling force at the edge of the hydrophobic thin film. We highlight that this capillary peeling process is more effective at lower velocities of the air-liquid interface and lower viscosities. Capillary peeling not only removes such thin films from the substrate but also transfers them flawlessly onto the air-water interface.

Laboratory layered latte

Inducing thermal gradients in fluid systems with initial, well-defined density gradients results in the formation of distinct layered patterns, such as those observed in the ocean due to double-diffusive convection. In contrast, layered composite fluids are sometimes observed in confined systems of rather chaotic initial states, for example, lattes formed by pouring espresso into a glass of warm milk. Here, we report controlled experiments injecting a fluid into a miscible phase and show that, above a critical injection velocity, layering emerges over a time scale of minutes. We identify critical conditions to produce the layering, and relate the results quantitatively to double-diffusive convection. Based on this understanding, we show how to employ this single-step process to produce layered structures in soft materials, where the local elastic properties vary step-wise along the length of the material.The ability to form density layering in a fluid in a simple and repeatable way is of relevance to a number of industrial and environmental processes. Here Xue et al. show formation of layers by simple injection of a hot liquid into a warm one at a predetermined critical pouring velocity.

Separation of particles by size from a suspension using the motion of a confined bubble

When confined in a liquid-filled circular cylinder, a long air bubble moves slightly faster than the bulk liquid as a small fraction of the liquid leaks through a very thin annular gap between the bubble and the internal wall of the cylinder. At low velocities, the thickness of this lubricating film formed around the bubble is set only by the liquid properties and the translational speed of the bubble and thus can be tuned in a simple fashion. Here, we use this setting to filter, based on size, micron-size particles that are originally dispersed in a suspension. Furthermore, we apply this process for separation of particles from a polydisperse solution. The bubble interface is free of particles initially, and particles of different sizes can enter the liquid film region. Particle separation occurs when the thickness of the lubricating liquid film falls between the diameters of the two different particles. While large particles will be collected at the bubble surface, smaller particles can leak through the thin film and reach the fluid region behind the bubble. As a result, the film thickness can be fine-tuned by simply adjusting the speed of a translating confined bubble, so as to achieve separation of particles by size based on the relative particle diameter compared to the film thickness.When confined in a liquid-filled circular cylinder, a long air bubble moves slightly faster than the bulk liquid as a small fraction of the liquid leaks through a very thin annular gap between the bubble and the internal wall of the cylinder. At low velocities, the thickness of this lubricating film formed around the bubble is set only by the liquid properties and the translational speed of the bubble and thus can be tuned in a simple fashion. Here, we use this setting to filter, based on size, micron-size particles that are originally dispersed in a suspension. Furthermore, we apply this process for separation of particles from a polydisperse solution. The bubble interface is free of particles initially, and particles of different sizes can enter the liquid film region. Particle separation occurs when the thickness of the lubricating liquid film falls between the diameters of the two different particles. While large particles will be collected at the bubble surface, smaller particles can leak through the...

Bacterial Biofilm Material Properties Enable Removal and Transfer by Capillary Peeling

Biofilms, surface-attached communities of bacterial cells, are a concern in health and in industrial operations because of persistent infections, clogging of flows, and surface fouling. Extracellular matrices provide mechanical protection to biofilm-dwelling cells as well as protection from chemical insults, including antibiotics. Understanding how biofilm material properties arise from constituent matrix components and how these properties change in different environments is crucial for designing biofilm removal strategies. Here, using rheological characterization and surface analyses of Vibrio cholerae biofilms, it is discovered how extracellular polysaccharides, proteins, and cells function together to define biofilm mechanical and interfacial properties. Using insight gained from our measurements, a facile capillary peeling technology is developed to remove biofilms from surfaces or to transfer intact biofilms from one surface to another. It is shown that the findings are applicable to other biofilm-forming bacterial species and to multiple surfaces. Thus, the technology and the understanding that have been developed could potentially be employed to characterize and/or treat biofilm-related infections and industrial biofouling problems.