A viscosity measurement technique for ultra-low sample volumes.

We describe a unique method to measure the viscosity of liquids based on the fluid mechanics of thin films. A drop of sample is spread over a substrate by contacting a blade with the drop and moving the blade across the substrate. The thickness of the film is determined by the capillary number, which measures the competition between the viscous force that smears the liquid over the glass slide and the surface tension that resists the deformation of the interface.

Film Formation of Iodinated Latex Dispersions and Its Role in Their Antimicrobial Activity.

Waterborne coatings with intrinsic antibacterial attributes have attracted significant attention due to their potential in mitigating microbial contamination while simultaneously addressing the environmental drawbacks of their solvent-based counterparts. Typically, antimicrobial coatings are designed to resist and eliminate microbial threats, encompassing challenges such as biofilm formation, fungal contamination, and proliferation of black mold. Iodine, when solubilized using ethylene glycol and incorporated as a complex into waterborne latex dispersions, has shown remarkable antimicrobial activity.

Critical cracking thickness of drying polymer films.

Polymer coatings are used for a number of applications such as for decorative purposes, to protect surfaces and as functional parts of devices. The mechanical integrity of the coatings is critical to their function and hence it is important that the coatings do not fail during their lifetime. Here, we present a simple model to determine conditions under which drying films of polymer solutions can crack.

Universality in the buckling behavior of drying suspension drops.

Fast evaporation of particle-suspension drops results in complex morphologies of the final dried granules. Understanding the morphological transformations is important to industrial processes such as spray drying where droplets of particulate suspensions are dried at a fast rate to produce granules of thermally sensitive materials. The transformation of an initial spherical shell to complex morphologies of the final dried granule has been attributed to the buckling of particle-packed shells.

Experimental Evaluation of the Impact of Rapid Environmental Changes on Stress Distribution in Tablet Coatings.

Drying-induced cracks in tablet coatings are undesirable as they not only affect tablet's appearance, but they may also interfere with its function. While it is well known that tensile stresses in the coating are responsible for coating failures, few have measured the stress in tablet coatings, especially when exposed to rapid environmental changes. In this study, two commercial tablet coatings based on Hydroxy Propyl Methyl Cellulose (HPMC) and Poly Vinyl Alcohol (PVA) are exposed to rapid variations in temperature and humidity to observe the variation in residual stress.

Impact of Rapid Environmental Changes on Stress Distribution in Tablet Coatings: Simulations.

Immediate-release film coatings, also known as "non-functional" film coating, are applied to core tablets to improve product appearance and swallowability, impart taste-masking properties, improve handling and stability of the dosage form, and reduce exposure to active drug substance for caregivers. The coatings have no measurable impact on bio-performance of the drug product but they protect tablets from negative effects of environment such as humidity, oxidation, and light. The mechanical stability and integrity of tablet coatings are therefore important to maintain drug product quality attributes such as appearance and stability.

A low-cost flow cell for flow cytometry.

Flow cytometry is an essential analytical technique used in biomedical diagnostics to measure properties of cells, micro-organisms, and particles. Laser light is scattered from particles focused in a flow cell and collected by light sensors, where the intensity of the scattered light is a function of the scattering angle, the refractive index of the particle and surrounding medium, the wavelength of light, and the size and the shape of the particle. One of the critical parts of the cytometer is the flow cell where the particle stream is constrained into a tight region within 10-30 μm using hydrodynamic focusing.

Moving cracks in drying colloidal films.

Drying colloidal films are encountered in many applications ranging from paints and coatings to ceramic and semiconductor processing. In many cases, shrinkage stresses are generated during drying, which can fracture the film. While much of the previous experimental and theoretical work has focused on cracking in static cracks, there are very few studies on the dynamics of cracks in colloidal coatings.

Osmotic tablet coatings: Drying stress, mechanical properties and microstructure.

The coatings in osmotic tablets play a critical role in controlling the release of active pharmaceutical ingredient. Coatings are formed by spraying dilute polymer solution onto the tablet surface. During drying, the films develop shrinkage stress, which can cause cracking.

Solventborne Polymer Coatings: Drying, Film Formation, Stress Evolution, and Failure.

Polymer coatings find use in a wide range of industrial applications, from conventional paints and coatings in building and construction to the pharmaceutical industry, organic solar cell production, and lithium battery technology. Despite their importance, there are gaps in our understanding of the drying process, the stress development during drying, and their influence on the final mechanical properties of the dried film. This perspective focuses on the fundamental aspects of the drying and film formation process, highlights the gaps, and suggests directions for future work.

Modeling the drying of polymer coatings.

We investigate the drying phenomenon in polymer coatings by developing a model that accounts for the polymer-lean phase (liquid) and the polymer-rich phase (solid), while predicting the stress in the coating. The governing equations are developed for the two phases separately. In the dilute polymer region, the effect of polymer diffusion on its concentration distribution is considered.

Ligand sensing enhances bacterial flagellar motor output via stator recruitment.

It is well known that flagellated bacteria, such as , sense chemicals in their environment by a chemoreceptor and relay the signals via a well-characterized signaling pathway to the flagellar motor. It is widely accepted that the signals change the rotation bias of the motor without influencing the motor speed. Here, we present results to the contrary and show that the bacteria is also capable of modulating motor speed on merely sensing a ligand.

Cracking in drying films of polymer solutions.

Thin films of polymer coatings have important industrial applications ranging from paints and coatings to pharmaceuticals. In many applications, the coatings are obtained by applying thin films of dilute polymer solutions, wherein the solvent evaporates to leave behind a thin polymer film. In some cases, the thin films may crack due to shrinkage stresses developed during drying.

Micro-mechanical theory of shear yield stress for strongly flocculated colloidal gel.

Shear yield stress is an important parameter in the processing of colloidal suspensions as it characterizes the solid-to-fluid transition. Although shear rheology of colloidal gel is of widespread academic and industrial interest, first principles theory that connects the microscopic properties to the macroscopic mechanical response in a self-consistent manner is lacking. In this work, we derive a constitutive relation to predict the yield stress for a strongly attractive gel undergoing quasi-static shear deformation as a function of volume fraction, inter-particle potential, contact scale properties and the micro-structure of a strongly-aggregated colloidal gel.

Capillary-Induced Motion of Particles Bridging Interfaces of a Free-Standing Thin Liquid Film.

We demonstrate a new form of capillary force experienced by neutrally buoyant spherical particles adsorbed simultaneously at both interfaces of a thin liquid film of spatially varying thickness. The force is proportional to the slope of the interface and the difference between the local contact angle and the equilibrium value, and exists even when the two bounding interfaces have zero curvature. We derive the expression for the force, which when balanced against the hydrodynamic drag gives the trajectory of the particle.

Buckling of a drying colloidal drop.

It is well known that drying drops of colloidal dispersions undergo complex morphological transitions involving buckling of a particle-packed outer shell during drying. Although capillary stresses generated during drying have been identified as the cause for buckling, the exact conditions for buckling and its relation to the particle size, rigidity, and nature of packing have not been understood. Here, we derive explicit expressions for the critical capillary pressure for buckling of droplets based on the mechanical properties of the particle network formed during drying and the conditions under which buckling can be avoided.

Dynamics of Radially Expanding Liquid Sheets.

The process of atomization often involves ejecting thin liquid sheets at high speeds from a nozzle that causes the sheet to flap violently and break up into fine droplets. The flapping of the liquid sheet has long been attributed to the sheet's interaction with the surrounding gas phase. Here, we present experimental evidence to the contrary and show that the flapping is caused by the thinning of the liquid sheet as it spreads out from the nozzle exit.

Free-standing monolayer films of ordered colloidal particles.

We report a novel method for the fabrication of large area, free standing monolayer films of close-packed colloidal particles. The method involves creating a free-standing, wet film of colloidal dispersion containing mono-dispersed hard particles (such as polystyrene or silica) mixed with smaller and softer polymer particles. During drying, hard particles present in the free standing film arrange in a hexagonal close-packed structure in a monolayer while the softer particles fill the interstices, and deform and coalesce to produce a continuous matrix around the hard particles.

Universality in consolidation of colloidal gels.

Consolidation of colloidal dispersions under external load is a complex process involving inter-particle interactions, thermal forces and hydrodynamics. Despite its importance in diverse industrial applications, past studies involving experiments, scaling approaches and simulations are yet to provide a comprehensive understanding of how the microstructure determines the mechanical response in three dimensional colloidal gels. Here, we develop a model that accounts for the microstructural details and predicts the mechanical response under slow, uniaxial compression of a strongly aggregated three dimensional colloidal gel.

Escherichia coli modulates its motor speed on sensing an attractant.

It is well known that Escherichia coli achieves chemotaxis by modulating the bias of the flagellar motor. Recent experiments have shown that the bacteria vary their swimming speeds as well in presence of attractants. However, this increase in the swimming speed in response to the attractants has not been correlated with the increase in the flagellar motor speed.

Dynamics of cracking in drying colloidal sheets.

Colloidal dispersions are known to display a fascinating network of cracks on drying. We probe the fracture mechanics of free-standing films of aqueous polymer-particle dispersions. Thin films of the dispersion are cast between a pair of plain steel wires and allowed to dry under ambient conditions.

Enhancement of Swimming Speed Leads to a More-Efficient Chemotactic Response to Repellent.

Negative chemotaxis refers to the motion of microorganisms away from regions with high concentrations of chemorepellents. In this study, we set controlled gradients of NiCl2, a chemorepellent, in microchannels to quantify the motion of Escherichia coli over a broad range of concentrations. The experimental technique measured the motion of the bacteria in space and time and further related the motion to the local concentration profile of the repellent.

Variation of swimming speed enhances the chemotactic migration of Escherichia coli.

Studies on chemotaxis of Escherichia coli have shown that modulation of tumble frequency causes a net drift up the gradient of attractants. Recently, it has been demonstrated that the bacteria is also capable of varying its runs speed in uniform concentration of attractant. In this study, we investigate the role of swimming speed on the chemotactic migration of bacteria.

Variation in swimming speed of Escherichia coli in response to attractant.

It is well known that Escherichia coli executes chemotactic motion in response to chemical cues by modulating the flagellar motor bias alone. However, previous studies have reported the possibility of variation in run speed in the presence of attractants although it is unclear whether bacteria can deliberately modulate their swimming speeds in response to environmental cues or if the motor speeds are hardwired. By studying the detailed motion of cells in a uniform concentration of glucose and its non-metabolizable analogue, we show that changing concentrations may be accompanied by variation in the swimming speed.

Asymptotic analysis of stresses near a crack tip in a two-dimensional colloidal packing saturated with liquid.

The consolidation of colloidal particles in drying colloidal dispersions is influenced by various factors such as particle size and shape, and interparticle potential. The capillary pressure induced by the menisci, formed between the top layer of particles in the packed bed, compresses the bed of particles while the constraints imposed by the boundaries result in tensile stresses in the packing. Presence of flaws or defects in the bed determines its ultimate strength under such circumstances.