Mahesh S. Tirumkudulu

Particle segregation in monodisperse sheared suspensions

It has been known for a long time that many mixtures of granular materials tend to segregate when tumbled in a rotating horizontal cylinder, with the different components separating into bands of relatively pure single concentration along the rotational axis [Mixing of Solids, Advances in Chemical Eng., edited by T. B. Drew and J. W. Hoopes (Academic Press, New York, 1952), Vol. 2, p. 211]. Here we report a phenomenon that seems to be analogous, but in suspensions of monodisperse neutrally buoyant spherical particles in a Newtonian liquid medium being sheared in a partially filled horizontal Couette device in which the suspension separates itself into alternating regions of high and low particle concentration along the length of the tube. The experiment is mostly qualitative, the aim at this stage being primarily to provide photographic evidence of a curious and as yet unexplained phenomenon.

Cracking in drying colloidal films of flocculated dispersions.

Understanding the mechanism of cracking during the drying of aqueous colloidal dispersions is important to preventing film failure. While most of the reported work has dealt with stable aqueous dispersions, a few studies have focused on flocculated systems. The latter especially assumes importance because the role of particle packing in the mechanism of cracking is not completely understood. In this work, we study the cracking of colloidal films cast from flocculated aqueous dispersions of alpha-alumina. Here, the extent of flocculation is controlled by varying the pH of the dispersion and characterized in terms of the final packing volume fraction of the dried film. The influence of varying the close-packed volume fraction on the critical cracking thickness and critical cracking stress is measured. The measurements are compared with the model predictions based on Griffiths energy balance, and good agreement is found between theory and experiments, suggesting that the model is universal and applies equally well to stable as well as flocculated systems.

Cracking in Soft−Hard Latex Blends: Theory and Experiments

Volatile organic compounds (VOCs) in the traditional paint and coating formulations are an important health and environmental concern, and current formulations are increasingly moving toward water-based dispersions. However, even within the water-based systems, small quantities of organic solvents are used to promote particle coalescence. One route to achieving this goal has been to use mixtures of soft and hard particles, also known as latex blends. We investigate the drying of colloidal films containing mixtures of silica and acrylic particles. Since both the particles deform only slightly at room temperature, this work investigates the cracking behavior of films containing elastic particles of two different elastic moduli. We extend an existing model for the stress versus strain relation for identical particles in a colloidal film to that containing a mixture of equal-sized hard and soft elastic spheres while accounting for the nonaffine deformation. A transition from soft to rigidlike behavior is observed beyond a critical hard particle volume fraction ratio that matches with published results obtained from computer simulations. The model predictions are validated with extensive experimental data on the critical stress and critical cracking thickness for various ratios of hard and soft particle volume fraction.

Chemotaxis of Escherichia coli to L-serine

A novel experimental technique was used to quantify the motion of E. coli to varying serine concentrations and gradients so as to capture the spatial and temporal variation of the chemotactic response. The average run speed and the cell diffusivity are found to be dependent on the serine concentration. The measured diffusivities were in the range of 1.2-2.5 x 10 (-10) m(2) s(-1). The study revealed that the rotational diffusivity of the cells, induced by the extracellular environment, also varies with the serine concentration. The drift velocity increased with serine gradients reaching a maximum value of approximately 5.5 microm s(-1) at 1.6 microM microm(-1) after which it decreased. Experimental analysis demonstrated the interdependence of run speed, rotational diffusivity and drift velocity that characterizes the motion. Further, the motion was found to critically depend on the oxygen concentration and energy level of the cells.

Consolidation of Charged Colloids during Drying

We consider the drying of latex dispersions containing submicrometer-sized particles dispersed in water. It is well known that the consolidation of colloidal particles is influenced by a number of factors such as particle size and shape and interparticle potential. In this work, we focus on the effect of surface charge on the consolidation front. Recent experimental and theoretical investigations on the sedimentation of charged colloidal spheres have shown that the large mass difference between noninteracting colloids and ions sets up a macroscopic electric field, thereby enhancing the diffusivity of the particles and resulting in an inflated sedimentation profile. Our experimental measurements of the concentration profile during drying-induced consolidation also reveal similar charge effects. We present a model for the consolidation of charged particles that accounts for the presence of an induced external electric field. As expected, the predicted particle diffusivity is enhanced by the onset of the electric field at low particle concentration. Fluorescence and bright-field microscopy were used to detect the particle concentration variation in a dispersion dried in a capillary, and the measured profile agrees with the prediction confirming the influence of particle charge on consolidation.

Measuring the “tack” of waterborne adhesives

Recent “tack” experiments on Newtonian liquids have shown that the force versus gap measurements obtained by pulling apart two surfaces separated by a thin liquid film result from a complex and dynamic balance between the viscous force resisting the separation and the spring force exerted by the compliant load cell of the instrument [Tirumkudulu et al. Phys. Fluids 15(6), 1588–1605 (2003)]. Here, we present similar experiments with waterborne adhesives that are colloidal dispersions of soft polymer spheres dispersed in water. By employing a simple power law to describe the complex rheology of the waterborne adhesives and a lubrication approximation for the viscous force, we predict pull-off forces in close agreement with those observed experimentally. However, experiments with high particle concentration adhesive and/or large separation rates cause cavitation in the gap, resulting in forces that are lower than predicted. Finally, a simple analysis for adhesives of two different rheologies shows that the tack test may result in contradictory conclusions, since the forces are sensitive to small variations in the instrument settings.

A novel methodology for measuring the tensile strength of expansive clays

Several experimental techniques (viz., triaxial tests, direct tensile tests or suction measurements) have been developed and employed by earlier researchers to measure the tensile strength of fine-grained soils. However, these studies yield results that are soil specific and dependent on the methodology adopted. Apart from this, due to the bulk form of the sample, the sample heterogeneity (i.e., both in terms of the density and the moisture content) influences test results to a great extent. Under this situation, and in the absence of guidelines regarding sample thickness, determination of the tensile strength of expansive clays by employing their thin samples (1 mm to 5 mm) appears to be an excellent alternative. With this in view, efforts were made to determine the tensile strength of expansive clays by resorting to techniques that deal with measurement of (a) the deflection undergone by a silicon wafer, due to air-drying of a thin film of these clays, with the help of a laser beam and (b) the suction by employing a dewpoint potentiameter, WP4. Results obtained from these techniques were critically evaluated vis-à-vis those obtained from triaxial tests and empirical relationships reported in the literature. It has been observed that there is a unique relationship between the results of the thin and thick samples and hence thin samples of expansive clays can be employed for determining their tensile strength. This shows the usefulness of the proposed techniques for measuring the tensile strength of expansive soils.

Mathematical modeling and experimental validation of chemotaxis under controlled gradients of methyl-aspartate in Escherichia coli

Escherichia coli has evolved an intracellular pathway to regulate its motion termed as chemotaxis so as to move towards a favorable environment such as regions with higher concentration of nutrients. Chemotaxis is a response to temporal and spatial variation of extracellular ligand concentration and randomness in motion induced by collisions with solvent molecules. Previous studies have reported average drift velocities for a given gradient and do not measure drift velocities as a function of time and space. To address this issue, a novel experimental technique was developed to quantify the motion of E. coli cells to varying concentrations and gradients of methyl-aspartate so as to capture the spatial and temporal variation of the drift velocity. A two-state receptor model accounting for the intracellular signaling pathway predicted the experimentally observed increase in drift velocity with gradient and the subsequent adaptation. Our study revealed that the rotational diffusivity induced by the extracellular environment is crucial in determining the drift velocity of E. coli. The model predictions matched with experimental observations only when the response of the intracellular pathway was highly ultra-sensitive to overcome the extracellular randomness. The parametric sensitivity of the pathway indicated that the dissociation constant for the binding of the ligand and the rate constants of the methylation/demethylation of the receptor are key to predict the performance of the chemotactic behavior. The study also indicates a possible role of oxygen in the chemotaxis response and that the response to a ligand may have to account for effects of oxygen.

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. In this study, we determine the asymptotic stress distribution around a flaw in a two-dimensional colloidal packing saturated with liquid and compare the results with those obtained from the full numerical solution of the problem. Using the Griffiths criterion for equilibrium cracks, we relate the critical capillary pressure at equilibrium to the crack size and the mechanical properties of the packed bed. The analysis also gives the maximum allowable flaw size for obtaining a crack-free packing.

Instability of a moving liquid sheet in the presence of acoustic forcing

The excitation of thin planar liquid sheets formed by impinging two collinear water jets to acoustic waves was studied at varying frequencies and sound pressure levels (SPLs). Experiments were conducted over a range of liquid velocities that encompassed the stable and flapping regimes of the sheet. For a given frequency, there was a threshold value of SPL below which the sheet was unaffected. The threshold SPL increased with frequency. Further, the sheet was observed to respond to a set of specific frequencies lying in the range of 100–300 Hz, the frequency set varying with the Weber number of the liquid sheet. The magnitude of the response for a fixed pressure level, characterized by the reduction in the extent of the sheet, was larger at lower frequencies. The droplet sizes formed by the disintegration of the sheet reduced with an increase in the measured response and the drop-shedding frequency was near the imposed frequency. Model equations for inviscid flow and accounting for the varying pressure fie...