Papers by Jonathan Rothstein
Soft Matter, 2009
A filament-stretching rheometer is used to measure the extensional properties of shear-thickening... more A filament-stretching rheometer is used to measure the extensional properties of shear-thickening nanoparticle suspensions as a function of concentration and extension rate. The experiments are performed using a series of colloidal suspensions consisting of concentrations of 17.5 wt%, 25 wt% and 30 wt% of fumed silica nanoparticles in polypropylene glycol. The shear rheology of these suspensions was found to demonstrate dynamic shear-thickening behavior owing to the formation of large hydrodynamic clusters. The critical value of angular frequency for the onset of shear-thickening was found to increase monotonically with decreased strain amplitude. The extensional rheology of all the tested suspensions demonstrated modest strain-hardening at low strain rates. At a critical extension rate, a dramatic increase in both the speed and magnitude of the strain-hardening is observed for both the 25 wt% and 30 wt% suspensions with increasing extensional rate. The steady state extensional viscosity as a function of extension rate shows sharp extensional thickening transition very similar to shear flows. The increase in strain-hardening is likely due to the formation of strings and clusters ordered in the flow direction. This hypothesis is confirmed by small-angle light scattering measurements of the flow of the nanoparticle suspension through a microfluidic hyperbolic contraction. The degree of alignment of nanoparticles is quantified from the analysis of the scattering patterns and found to increase significantly with increasing extension rate.
A series of experiments were performed to further investigate the phenomenon of shear-banding in ... more A series of experiments were performed to further investigate the phenomenon of shear-banding in surfactant solutions. Many surfactant solutions, through their unique amphiphilic chemistry, form long wormlike micelle structures which behave like living polymers. These wormlike micelles have interesting viscoelastic properties and have been the subject of a number of recent studies. These water-based surfactant systems are widely used in
Previous experimental measurements and linear stability analyses of curvilinear shearing flows of... more Previous experimental measurements and linear stability analyses of curvilinear shearing flows of viscoelastic fluids have shown that the combination of streamwise curvature and elastic normal stresses can lead to flow destabilization. Torsional shear flows of highly elastic fluids with closed streamlines can also accumulate heat from viscous dissipation resulting in nonuniformity in the temperature profile within the flow and nonlinearity in the viscometric properties of the fluid. Recently, it has been shown by Al-Mubaiyedh et al. ͓Phys. Fluids 11, 3217 ͑1999͔͒ that the inclusion of energetics in the linear stability analysis of viscoelastic Taylor-Couette flow can change the dominant mode of the purely elastic instability from a nonaxisymmetric and time-dependent secondary flow to an axisymmetric stationary Taylor-type toroidal vortex that more closely agrees with the stability characteristics observed experimentally. In this work, we present a detailed experimental study of the effect of viscous heating on the torsional steady shearing of elastic fluids between a rotating cone and plate and between two rotating coaxial parallel plates. Elastic effects in the flow are characterized by the Deborah number, De, while the magnitude of the viscous heating is characterized by the Nahme-Griffith number, Na. We show that the relative importance of these two competing effects can be quantified by a new dimensionless thermoelastic parameter, ⌰ϭNa 1/2 /De, which is a material property of a given viscoelastic fluid independent of the rate of deformation. By utilizing this thermoelastic number, experimental observations of viscoelastic flow stability in three different fluids and two different geometries over a range of temperatures can be rationalized and the critical conditions unified into a single flow stability diagram. The thermoelastic number is a function of the molecular weight of the polymer, the flow geometry, and the temperature of the test fluid. The experiments presented here were performed using test fluids consisting of three different high molecular weight monodisperse polystyrene solutions in various flow geometries and over a large range of temperatures. By systematically varying the temperature of the test fluid or the configuration of the test geometry, the thermoelastic number can be adjusted appreciably. When the characteristic time scale for viscous heating is much longer than the relaxation time of the test fluid ͑⌰Ӷ1͒ the critical conditions for the onset of the elastic instability are in good agreement with the predictions of isothermal linear stability analyses. As the thermoelastic number approaches a critical value, the strong temperature gradients induced by viscous heating reduce the elasticity of the test fluid and delay the onset of the instability. At even larger values of the thermoelastic parameter, viscous heating stabilizes the flow completely.
A study of the confinement effects on chain entanglements in free-standing ultrathin (<100 nm)... more A study of the confinement effects on chain entanglements in free-standing ultrathin (<100 nm) polymer films is presented. Chain entanglements are probed by determining the lifetime and breakup time scale of a branched network of suspended fibers formed from the annealing of these films. Films of polystyrene (between 50 and 100 nm) cast via flow coating are suspended atop lithographically patterned arrays of pillars. The films are then annealed above the glass transition temperature, where holes are randomly formed. The holes expand exponentially due to capillary forces and impinge upon each other to form a suspended, branched network of fibers. The thinning of fibers as well as the lifetime and breakup of this fiber network is observed via optical microscopy. A model for the viscoelastic-capillary thinning of fibers can be applied to determine a time scale for the breakup of individual samples. The decay of this time scale, below a critical parent film thickness, shows a transit...
We investigate the formation and stability of spherical and cylindrical droplets of immiscible fl... more We investigate the formation and stability of spherical and cylindrical droplets of immiscible fluids undergoing flow. Our device employs a hydrodynamic-focusing geometry that we created using glass fibers in a coaxial arrangement. We research the stability of flowing non-Newtonian aqueous fluids surrounded by a co-flowing oil phase. We use water with polyacrylamide for the inner non-Newtonian fluid and hexadecane for the outer fluid. The morphology of the stream and droplets are controlled by varying the flow rates of the oil and aqueous fluids independently. We also explore the stabilization of the liquid structures by adsorption of microscopic colloidal particles at the interface, which create a rigid shell on the surface of the cylinders. The results are expected to be relevant to micro-fluidic systems with non-Newtonian fluids. We acknowledge support from Kraft Foods, Inc. NanoteK Consortium.
The interaction between solid surfaces and liquids is of fundamental importance in engineering fl... more The interaction between solid surfaces and liquids is of fundamental importance in engineering flows where solid surfaces are the primary means for controlling or manipulating fluids. We will show that by treating a solid surface to make it ultrahydrophobic it is possible to significantly enhance how the surface interacts with a flowing liquid. In particular, in our recent work, we have shown that ultrahydrophobic surfaces can be utilized to reduce drag [1, 2] and enhance mixing [3] in low to moderate Reynolds number internal flows. In this presentation we will demonstrate through a series of numerical simulations that ultrahydrophobic surfaces can also be used to delay the transition to turbulence and dramatically reduce drag in both external and internal turbulent flows. Ultrahydrophobic surfaces are engineered by taking materials with micron or nanoscale surfaces roughness and chemically treating them to make them hydrophobic. Because of the hydrophobicity of these microscale and...
Interface tracking techniques, used in conjunction with fin ite volume methods to study droplet f... more Interface tracking techniques, used in conjunction with fin ite volume methods to study droplet formation, have gained recent popularity, particularly at low and moderate Weber numbers. Simulations of interface behavior using moving and deforming meshes are particularly difficult, sin ce the quality of the underlying mesh must be maintained to obtain high solution accuracy. This paper addresses a few of
Scientific Reports, 2015
The condition of heat transfer to lignocellulosic biomass particles during thermal processing at ... more The condition of heat transfer to lignocellulosic biomass particles during thermal processing at high temperature (>400 °C) dramatically alters the yield and quality of renewable energy and fuels. In this work, crystalline cellulose particles were discovered to lift off heated surfaces by high speed photography similar to the Leidenfrost effect in hot, volatile liquids. Order of magnitude variation in heat transfer rates and cellulose particle lifetimes was observed as intermediate liquid cellulose droplets transitioned from low temperature wetting (500-600 °C) to fully de-wetted, skittering droplets on polished surfaces (>700 °C). Introduction of macroporosity to the heated surface was shown to completely inhibit the cellulose Leidenfrost effect, providing a tunable design parameter to control particle heat transfer rates in industrial biomass reactors.
Direct interface tracking has recently matured to where it is a useful tool for simulating the fu... more Direct interface tracking has recently matured to where it is a useful tool for simulating the fundamental physics of jet evolution and jet formation. This technique is anticipated to be very helpful in understanding the physics of ink-jet printing, where increased ink concentrations will create significant non-Newtonian behavior. A revised scheme for simulating the growth and pinching of a non-Newtonian liquid jet is described and demonstrated in this paper. Similar to some past efforts, the mesh moves with the liquid interface in a Lagrangian manner. There is no smearing or reconstruction of the interface. In the interior of the fluids, away from the interface, the vertex motion is calculated using a decomposition technique based on a finite-element method. The equations that govern the mesh motion are calculated quite cheaply using a conjugate-gradient solver. The conservation of mass, momentum, and energy are solved for the general case of moving, deform- ing control volumes. Th...
A filament-stretching rheometer is used to measure the extensional viscosity of a shear-thickenin... more A filament-stretching rheometer is used to measure the extensional viscosity of a shear-thickening suspension of cornstarch in water. The experiments are performed at a concentration of 55 wt.%. The shear rheology of these suspensions demonstrates a strong shear-thickening behavior. The extensional rheology of the suspensions demonstrates a Newtonian response at low extension rates. At moderate strain rates, the fluid strain hardens. The speed of the strain hardening and the extensional viscosity achieved increase quickly with increasing extension rate. Above a critical extension rate, the extensional viscosity goes through a maximum and the fluid filaments fail through a brittle fracture at a constant tensile stress. The glassy response of the suspension is likely the result of jamming of particles or clusters of particles at these high extension rates. This same mechanism is responsible for the shear thickening of these suspensions. In capillary breakup extensional rheometry, measurement of these suspensions demonstrates a divergence in the extensional viscosity as the fluid stops draining after a modest strain is accumulated.
The effect of initial microstructural deformation, alignment, and morphology on the response of w... more The effect of initial microstructural deformation, alignment, and morphology on the response of wormlike micelle solutions in transient uniaxial extensional flows is investigated using a pre-shear device attached to a filament stretching rheometer. In filament stretching experiments, increasing the strength and the duration of the pre-shear just before stretch is found to delay the onset of strain hardening. In these experiments, the wormlike micelle solution filaments fail through a rupture near the axial midplane. The value of the elastic tensile stress at rupture is found to decrease with increasing pre-shear rate and duration. The most dramatic effects are observed at shear rates for which shear banding has been independently observed. The reduction in the strain hardening suggests that pre-shear before filament stretching might break down the wormlike micelles reducing their size before stretch. Strain hardening is also observed in capillary breakup rheometry experiments; however, the pre-sheared wormlike micelle solutions strain harden faster, achieve larger steadystate extensional viscosities and an increase in the extensional relaxation time with increasing shear rate and duration. The difference between the response of the wormlike micelles in filament stretching and capillary breakup experiments demonstrates the sensitivity of these self-assembling micelle networks to pre-conditioning.
Rheologica Acta, 2009
Extensional rheometry has only recently been developed into a commercially available tool with th... more Extensional rheometry has only recently been developed into a commercially available tool with the introduction of the capillary breakup extensional rheometer (CaBER). CaBER is currently being used to measure the transient extensional viscosity evolution of mid to low-viscosity viscoelastic fluids. The elegance of capillary breakup extensional experiments lies in the simplicity of the procedure. An initial step-stretch is applied to
At a critical output rate, most extruded polymers exhibit a surface instability referred to as sh... more At a critical output rate, most extruded polymers exhibit a surface instability referred to as sharkskin. Sharkskin is typically unacceptable for industry and has motivated research to characterize the nature of the instability and delay its onset to higher stress levels and output rates. We present experimental work performed using a simple extruder and a series of circular and rectangular cross section capillaries which incorporate a thermal break. The low thermal conductivity of these fluids makes it possible for localized surface heating to induce a precise temperature gradient resulting in a stratification of rheological properties. Our measurements demonstrate that the surface instability is independent of gradients in the bulk fluid properties, and that the amplitude and frequency of the surface can be correlated directly to the rheological properties of the polymer melt evaluated at the surface temperature. By controlling the instability with isolated heating of the extrudat...
Rheologica Acta, 2007
The effect of initial microstructural deformation, alignment, and morphology on the response of w... more The effect of initial microstructural deformation, alignment, and morphology on the response of wormlike micelle solutions in transient uniaxial extensional flows is investigated using a pre-shear device attached to a filament stretching rheometer. In filament stretching experiments, increasing the strength and the duration of the pre-shear just before stretch is found to delay the onset of strain hardening. In these experiments, the wormlike micelle solution filaments fail through a rupture near the axial midplane. The value of the elastic tensile stress at rupture is found to decrease with increasing pre-shear rate and duration. The most dramatic effects are observed at shear rates for which shear banding has been independently observed. The reduction in the strain hardening suggests that pre-shear before filament stretching might break down the wormlike micelles reducing their size before stretch. Strain hardening is also observed in capillary breakup rheometry experiments; however, the pre-sheared wormlike micelle solutions strain harden faster, achieve larger steadystate extensional viscosities and an increase in the extensional relaxation time with increasing shear rate and duration. The difference between the response of the wormlike micelles in filament stretching and capillary breakup experiments demonstrates the sensitivity of these self-assembling micelle networks to pre-conditioning.
Rheologica Acta, 2010
A filament-stretching rheometer is used to measure the extensional viscosity of a shear-thickenin... more A filament-stretching rheometer is used to measure the extensional viscosity of a shear-thickening suspension of cornstarch in water. The experiments are performed at a concentration of 55 wt.%. The shear rheology of these suspensions demonstrates a strong shear-thickening behavior. The extensional rheology of the suspensions demonstrates a Newtonian response at low extension rates. At moderate strain rates, the fluid strain hardens. The speed of the strain hardening and the extensional viscosity achieved increase quickly with increasing extension rate. Above a critical extension rate, the extensional viscosity goes through a maximum and the fluid filaments fail through a brittle fracture at a constant tensile stress. The glassy response of the suspension is likely the result of jamming of particles or clusters of particles at these high extension rates. This same mechanism is responsible for the shear thickening of these suspensions. In capillary breakup extensional rheometry, measurement of these suspensions demonstrates a divergence in the extensional viscosity as the fluid stops draining after a modest strain is accumulated.
Rheologica Acta, 2009
Extensional rheometry has only recently been developed into a commercially available tool with th... more Extensional rheometry has only recently been developed into a commercially available tool with the introduction of the capillary breakup extensional rheometer (CaBER). CaBER is currently being used to measure the transient extensional viscosity evolution of mid to low-viscosity viscoelastic fluids. The elegance of capillary breakup extensional experiments lies in the simplicity of the procedure. An initial step-stretch is applied to generate a fluid filament. What follows is a self-driven uniaxial extensional flow in which surface tension is balanced by the extensional stresses resulting from the capillary thinning of the liquid bridge. In this paper, we describe the results from a series of experiments in which the step-stretch parameters of final length, and the extension rate of the stretch were varied and their effects on the measured extensional viscosity and extensional relaxation time were recorded. To focus on the parameter effects, well-characterized surfactant wormlike micelle solutions, polymer solutions, and immiscible polymer blends were used to include a range of characteristic relaxation times and morphologies. Our experimental results demonstrate a strong dependence of extensional rheology on step-stretch conditions for both wormlike micelle solutions and immiscible polymer blends. Both the extensional viscosity and extensional relaxation time of the wormlike micelle solutions were found to decrease with increasing extension rate and strain of the step-stretch. For the case of the immiscible polymer blends, fast step-stretches were found to result in droplet deformation and an overshoot in the extensional viscosity which increased with increasing strain rates. Conversely, the polymer solutions tested were found to be insensitive to step-stretch parameters. In addition, numerical simulations were performed using the appropriate constitutive models to assist in both the interpretation of the CaBER results and the optimization of the experimental protocol. From our results, it is clear that any rheological results obtained using the CaBER technique must be properly considered in the context of the stretch parameters and the effects that preconditioning has on viscoelastic fluids.
Physics of Fluids, 2004
Superhydrophobic surfaces combine hydrophobic surface chemistry with topological microfeatures. T... more Superhydrophobic surfaces combine hydrophobic surface chemistry with topological microfeatures. These surfaces have been shown to provide drag reduction in laminar and turbulent flows. In this work, direct numerical simulation is used to investigate the drag reducing performance of superhydrophobic surfaces in turbulent channel flow. Slip velocities, wall shear stresses, and Reynolds stresses are determined for a variety of superhydrophobic surface microfeature geometry configurations at friction Reynolds numbers of Re Ϸ 180, Re Ϸ 395, and Re Ϸ 590. This work provides evidence that superhydrophobic surfaces are capable of reducing drag in turbulent flow situations by manipulating the laminar sublayer. For the largest microfeature spacing, an average slip velocity over 80% of the bulk velocity is obtained, and the wall shear stress reduction is found to be greater than 50%. The simulation results suggest that the mean velocity profile near the superhydrophobic wall continues to scale with the wall shear stress and the log layer is still present, but both are offset by a slip velocity that is primarily dependent on the microfeature spacing.
Physics of Fluids, 2005
A series of experiments are presented which study the flow kinematics of water past drag-reducing... more A series of experiments are presented which study the flow kinematics of water past drag-reducing superhydrophobic surfaces. The ultrahydrophobic surfaces are fabricated from silicon wafers using photolithography and are designed to incorporate precise patterns of micrometer-sized ridges aligned in the flow direction. The ridges are made hydrophobic through a chemical reaction with an organosilane. An experimental flow cell is used to measure the velocity profile and the pressure drop as a function of the flow rate for a series of rectangular cross-section microchannel geometries and ultrahydrophobic surface designs. The velocity profile across the microchannel is determined through microparticle image velocimetry ͑-PIV͒ measurements capable of resolving the flow down to lengthscales well below the size of the surface features. Through these detailed velocity measurements, it is demonstrated that slip along the shear-free air-water interface supported between the hydrophobic micrometer-sized ridges is the primary mechanism responsible for the drag reduction observed for flows over ultrahydrophobic surfaces. A maximum slip velocity of more than 60% of the average velocity in the microchannel is found at the center of the shear-free air-water interface whereas the no-slip boundary condition is found to hold along the surface of the hydrophobic ridges. The experimental velocity and pressure drop measurements are compared to the predictions of numerical simulations and an analytical theory based on a simple model of an ultrahydrophobic surface composed of alternating shear-free and no-slip bands with good agreement.
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Papers by Jonathan Rothstein