Arshad Kudrolli - Academia.edu (original) (raw)
Papers by Arshad Kudrolli
Physics of Fluids
We demonstrate conditions that give rise to cave-like features commonly found in dissolving cliff... more We demonstrate conditions that give rise to cave-like features commonly found in dissolving cliffsides with a minimal two-phase physical model. Alcoves that are wider at the top and tapered at the bottom, with sharp-edged ceilings and sloping floors, are shown to develop on vertical solid surfaces dissolving in aqueous solvents. As evident from descending plumes, sufficiently large indentations evolve into alcoves as a result of the faster dissolution of the ceiling due to a solutal Rayleigh–Bénard density inversion instability. In contrast, defects of size below the boundary layer thickness set by the critical Rayleigh number smooth out, leading to stable planar interfaces. The ceiling recession rate and the alcove opening area evolution are shown to be given to first-order by the critical Rayleigh number. By tracking passive tracers in the fluid phase, we show that the alcoves are shaped by the detachment of the boundary layer flow and the appearance of a pinned vortex at the lead...
Submitted for the MAR09 Meeting of The American Physical Society Aggregation of athermal particle... more Submitted for the MAR09 Meeting of The American Physical Society Aggregation of athermal particles induced by capillarity MICHAEL BERHANU, ARSHAD KUDROLLI, Department of Physics, Clark University, Worcester, MA 01610. — Aggregation of cohesive particles floating in a medium is a very broad physical phenomena occurring in colloidal systems, soot particles, and intergalactic dust under gravitation. We investigate the geometrically constrained dynamics of aggregation with new experiments using floating spheres at the air-liquid interface. A short range attractive force can be induced by careful choice of buoyancy and capillarity to create self-assembled particle structures which can be tracked by imaging. First, the particles are placed randomly at the interface, and then aggregation is induced by smoothly decreasing the area of the interface which causes the particles to come within the attractive force range caused by capillarity. We measure the area fraction at which the connectivit...
arXiv: Soft Condensed Matter, 2020
Twisting sheets as a strategy to form yarns with nested structure lacks scientific guiding princi... more Twisting sheets as a strategy to form yarns with nested structure lacks scientific guiding principles but relies on millennia of human experience in making catguts, food packaging, and redeployable fabric wearables. We formulate a tensional twist-folding route to making yarns with prescribed folded, scrolled, and encapsulated architectures by remote boundary loading. By harnessing micro-focus x-ray scanning to noninvasively image the fine internal structure, we show that a twisted sheet follows a surprisingly ordered folding transformation as it self-scrolls to form structured yarns. As a sheet is twisted by a half-turn, we find that the elastic sheet spiral accordion folds with star polygon shapes characterized by Schlafli symbols set by the primary instability. A scalable model incorporating dominant stretching modes with origami kinematics explains not only the observed multilayered structure, torque, and energetics, but also the topological transformation into yarns with prescri...
Bulletin of the American Physical Society, 2011
... Authors: Moumita Dasgupta (Clark University). ... We will further discuss the dependence of s... more ... Authors: Moumita Dasgupta (Clark University). ... We will further discuss the dependence of swimming speed on Deborah number and other characteristics of the fluid. To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2011.MAR.Y9.6. ...
BACKGROUND: Exhalation exposure from patients to healthcare workers (HCWs), while using a nasal c... more BACKGROUND: Exhalation exposure from patients to healthcare workers (HCWs), while using a nasal cannula or simple O2 mask used in treating COVID-19 and other respiratory diseases, is a present and future risk. Little is known on exhalation dispersal through these devices, and on mitigating the viral exposure to those in the vicinity. METHODS: Respiration through O2 therapy devices was studied with a supine manikin equipped with a controllable mechanical lung by measuring aerosol density and flow with direct imaging. Dispersal direction and distances were quantified while placing a surgical mask loosely over the devices and contrasted with unmitigated oxygenation device use. Exhalation jets were examined over the entire range of oxygenation rates used in treatment. RESULTS: Exhalation jets travel 0.35 plus/minus 0.02 m upward while wearing a nasal cannula, and 0.29 plus/minus 0.02 m laterally while wearing a simple O2 mask posing significant inhalation risk. Placing a surgical facema...
Bulletin of the American Physical Society, 2012
Submitted for the DFD12 Meeting of The American Physical Society Speed of a Taylor Swimmer in New... more Submitted for the DFD12 Meeting of The American Physical Society Speed of a Taylor Swimmer in Newtonian and Viscoelastic Fluids MOUMITA DASGUPTA, Clark University, BIN LIU, Brown University, HENRY FU, University of Nevada, Reno, MICHAEL BERHANU, MSC Paris Diderot University, KENNETH BREUER, THOMAS POWERS, Brown University, ARSHAD KUDROLLI, Clark University — We demonstrate that the speed of an idealized Taylor swimmer with a prescribed waveform in a viscoelastic fluid can be greater or lesser than in a Newtonian fluid depending on their rheological properties. The measurements are performed using a cylindrical sheet immersed in a fluid inside a cylindrical tank under torque free conditions with traveling waves imposed in the azimuthal direction. Swimming speeds in the Newtonian case are found to be consistent with calculations using the Stokes equation. A faster swimming speed is found in a viscoelastic fluid which has a constant viscosity with shear rate. By contrast, a slower swim...
ABSTRACT Living organisms exhibit remarkable cooperative behavior in response to environmental st... more ABSTRACT Living organisms exhibit remarkable cooperative behavior in response to environmental stress. Bacterial colonies growing on a nutrient-rich substrate have served as model systems for studying pattern formation and population dynamics in biological systems. We report a novel spatio-temporal response of a Bacillus subtilis colony initially growing under ambient conditions to ultraviolet (UV) radiation exposure. The growth of the colonies decreased, but remarkably, the bacteria at the center of the colonies migrated towards the colony edge forming a ring during uniform exposure. The migration of the bacteria has no obvious benefit since they are still under UV radiation of the same strength. When the radiation is switched off, the colonies were observed to grow both inward into the evacuated regions as well as outward indicating that the pattern is not a swarming ring formed due to depletion of nutrients at the center of the colony. We also discuss the results of directed random walk simulations. Using simple models we show the rearrangement is not a consequence of linear attraction or repulsion between the bacteria.
ABSTRACT We study the effect of liquids on the angle of inclination of a wet sandpile in a rotati... more ABSTRACT We study the effect of liquids on the angle of inclination of a wet sandpile in a rotating drum system. In this system, the surface exhibits stick slip motion for slow rotation rates omega, and continuos avalanching above a critical omega. We will focus on the stick-slip regime, where the angle of inclination of the pile oscillates between the maximum angle of stability before an avalanche, and the angle of repose after the avalanche. Both angles are observed to increase and saturate as a function of the volume fraction of the fluid. Furthermore, by changing the viscosity of the fluid using water-glycerol mixtures, we observe that both the maximum angle of stability and the angle of repose increase with the viscosity of the fluid. There are two possible explanations for the increase of the angle of stability of the pile. First, there may be creep motion between the particles giving rise to viscous forces before an avalanche, that are too small to observe visually. The creep motion is also slower than the rate of increase of the surface due to rotation. Second, the average number of liquid bridges between particles may increase with viscosity. We will comment on both possibilities using experimental data and scaling arguments.
Submitted for the MAR06 Meeting of The American Physical Society Curvature condensation and twinn... more Submitted for the MAR06 Meeting of The American Physical Society Curvature condensation and twinning in an indented elastic shell MOUMITA DAS, ASHKAN VAZIRI, Harvard University, ARSHAD KUDROLLI, Clark University, L. MAHADEVAN, Harvard University — We study the formation of a localized geometrical defect and its evolution in an elastic shell using a combination of experiment and numerical simulation. We find that as a symmetric localized indentation on a semi-cylindrical shell increases, there is a transition from a global mode of deformation to a localized one which leads to the condensation of curvature along a parabolic crease along the line of symmetry. Further indentation leads to a twinning phenomena wherein the crease bifurcates into two defects that move apart on either side of the line of symmetry. We present a simple theory to explain the main features of the experiments and numerical simulations. Moumita Das University of California, Los Angeles Date submitted: 21 Nov 2005...
We investigate the erosive growth of channels in a thin subsurface sedimentary layer driven by hy... more We investigate the erosive growth of channels in a thin subsurface sedimentary layer driven by hydrodynamic drag toward understanding subterranean networks and their relation to river networks charged by ground water. Building on a model based on experimental observations of fluid-driven evolution of bed porosity, we focus on the characteristics of the channel growth and their bifurcations in a horizontal rectangular domain subject to various fluid source and sink distributions. We find that the erosion front between low- and high-porosity regions becomes unstable, giving rise to branched channel networks, depending on the spatial fluctuations of the fluid flow near the front and the degree to which the flow is above the erodibility threshold of the medium. Focusing on the growth of a network starting from a single channel, and by identifying the channel heads and their branch points, we find that the number of branches increases sublinearly and is affected by the source distributio...
We investigate mucosalivary dispersal and deposition on horizontal surfaces corresponding to huma... more We investigate mucosalivary dispersal and deposition on horizontal surfaces corresponding to human exhalations with physical experiments under still-air conditions. Synthetic fluorescence tagged sprays with size and speed distributions comparable to human sneezes are observed with high-speed imaging. We show that while some larger droplets follow parabolic trajectories, smaller droplets stay aloft for several seconds and settle slowly with speeds consistent with a buoyant cloud dynamics model. The net deposition distribution is observed to become correspondingly broader as the source height H is increased, ranging from sitting at a table to standing upright. We find that the deposited mucosaliva decays exponentially in front of the source, after peaking at distance x = 0.71 m when H = 0.5 m, and x = 0.56 m when H = 1.5 m, with standard deviations ≈0.5 m. Greater than 99% of the mucosaliva is deposited within x = 2 m, with faster landing times further from the source. We then demonst...
Physical Review Letters
We investigate the development of mobility inversion and fingering when a granular suspension is ... more We investigate the development of mobility inversion and fingering when a granular suspension is injected radially between horizontal parallel plates of a cell filled with a miscible fluid. While the suspension spreads uniformly when the suspension and the displaced fluid densities are exactly matched, even a small density difference is found to result in a dense granular front which develops fingers with angular spacing that increase with granular volume fraction and decrease with injection rate. We show that the time scale over which the instability develops is given by the volume fraction dependent settling time scale of the grains in the cell. We then show that the mobility inversion and the non-equilibrium Korteweg surface tension due to granular volume fraction gradients determine the number of fingers at the onset of the instability in these miscible suspensions.
Proceedings of the National Academy of Sciences
We investigate the dynamics ofLumbriculus variegatusin water-saturated sediment beds to understan... more We investigate the dynamics ofLumbriculus variegatusin water-saturated sediment beds to understand limbless locomotion in the benthic zone found at the bottom of lakes and oceans. These slender aquatic worms are observed to perform elongation–contraction and transverse undulatory strokes in both water-saturated sediments and water. Greater drag anisotropy in the sediment medium is observed to boost the burrowing speed of the worm compared to swimming in water with the same stroke using drag-assisted propulsion. We capture the observed speeds by combining the calculated forms based on resistive-force theory of undulatory motion in viscous fluids and a dynamic anchor model of peristaltic motion in the sediments. Peristalsis is found to be effective for burrowing in noncohesive sediments which fill in rapidly behind the moving body inside the sediment bed. Whereas the undulatory stroke is found to be effective in water and in shallow sediment layers where anchoring is not possible to a...
Physical Review E
We measure the drag encountered by a vertically oriented rod moving across a sedimented granular ... more We measure the drag encountered by a vertically oriented rod moving across a sedimented granular bed immersed in a fluid under steady-state conditions. At low rod speeds, the presence of the fluid leads to a lower drag because of buoyancy, whereas a significantly higher drag is observed with increasing speeds. The drag as a function of depth is observed to decrease from being quadratic at low speeds to appearing more linear at higher speeds. By scaling the drag with the average weight of the grains acting on the rod, we obtain the effective friction µe encountered over six orders of magnitude of speeds. While a constant µe is found when the grain size, rod depth and fluid viscosity are varied at low speeds, a systematic increase is observed as the speed is increased. We analyze µe in terms of the inertial number I and viscous number J to understand the relative importance of inertia and viscous forces, respectively. For sufficiently large fluid viscosities, we find that the effect of varying the speed, depth, and viscosity can be described by the empirical function µe = µo + kJ n , where µo is the effective friction measured in the quasi-static limit, and k and n are material constants. The drag is then analyzed in terms of the effective viscosity ηe and found to decrease systematically as a function of J. We further show that ηe as a function of J is directly proportional to the fluid viscosity and the µe encountered by the rod.
Physical Review Fluids
We investigate the effective friction encountered by an intruder moving through a sedimented medi... more We investigate the effective friction encountered by an intruder moving through a sedimented medium which consists of transparent granular hydrogels immersed in water, and the resulting motion of the medium. We show that the effective friction µ e on a spherical intruder is captured by the inertial number I given by the ratio of the time scale over which the intruder moves and the inertial time scale of the granular medium set by the overburden pressure. Further, µ e is described by the function µ e (I) = µ s + αI β , where µ s is the static friction, and α and β are material dependent constants which are independent of intruder depth and size. By measuring
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science
We investigate with experiments the twist-induced transverse buckling instabilities of an elastic... more We investigate with experiments the twist-induced transverse buckling instabilities of an elastic sheet of length L , width W and thickness t , that is clamped at two opposite ends while held under a tension T . Above a critical tension T λ and critical twist angle η tr , we find that the sheet buckles with a mode number n ≥1 transverse to the axis of twist. Three distinct buckling regimes characterized as clamp-dominated, bendable and stiff are identified, by introducing a bendability length L B and a clamp length L C (< L B ). In the stiff regime ( L > L B ), we find that mode n =1 develops above η tr ≡ η S ∼( t / W ) T −1/2 , independent of L . In the bendable regime L C < L < L B , n =1 as well as n >1 occur above η tr ≡ η B ∼ t / L T − 1 / 4 . Here, we find the wavelength λ B ∼ L t T − 1 / 4 , when n >1. These scalings agree with those derived from a covariant form of the Föppl-von Kármán equations, however, we find that the n =1 mode also occurs over a surpri...
Physical Review E
We investigate the deformation of a longitudinally stretched rectangular sheet which is clamped a... more We investigate the deformation of a longitudinally stretched rectangular sheet which is clamped at two opposite boundaries and free otherwise with experiments, numerical analysis and asymptotic analysis of the biharmonic elastic equation governing their planar equilibrium configurations. The displacement field of the sheet is measured by tracking embedded fluorescent tracers with a digital image correlation (DIC) technique. The experiments and numerical finite element analysis (FEA) are found to be in overall good agreement except at the very corners where large deformations occur. We find that the deformed sheet can be broadly divided into a uniaxially stretched central region and two clamp dominated side regions. A subregion characterized by a diverging stress can be identified at each of the four clamped-free corners within the clamp dominated region. We postulate that the divergence at the corners is regularized by nonlinear elastic deformations occurring in this subregion at the very corners and provide a nontrivial scaling for its size. Within the intermediate corner dominated subregion, measured displacements grow with distance r from the corners as r α , with power α < 1 consistent with the development of stress singularities at the intersection of the free and clamped edges.
Physics of Fluids
We demonstrate conditions that give rise to cave-like features commonly found in dissolving cliff... more We demonstrate conditions that give rise to cave-like features commonly found in dissolving cliffsides with a minimal two-phase physical model. Alcoves that are wider at the top and tapered at the bottom, with sharp-edged ceilings and sloping floors, are shown to develop on vertical solid surfaces dissolving in aqueous solvents. As evident from descending plumes, sufficiently large indentations evolve into alcoves as a result of the faster dissolution of the ceiling due to a solutal Rayleigh–Bénard density inversion instability. In contrast, defects of size below the boundary layer thickness set by the critical Rayleigh number smooth out, leading to stable planar interfaces. The ceiling recession rate and the alcove opening area evolution are shown to be given to first-order by the critical Rayleigh number. By tracking passive tracers in the fluid phase, we show that the alcoves are shaped by the detachment of the boundary layer flow and the appearance of a pinned vortex at the lead...
Submitted for the MAR09 Meeting of The American Physical Society Aggregation of athermal particle... more Submitted for the MAR09 Meeting of The American Physical Society Aggregation of athermal particles induced by capillarity MICHAEL BERHANU, ARSHAD KUDROLLI, Department of Physics, Clark University, Worcester, MA 01610. — Aggregation of cohesive particles floating in a medium is a very broad physical phenomena occurring in colloidal systems, soot particles, and intergalactic dust under gravitation. We investigate the geometrically constrained dynamics of aggregation with new experiments using floating spheres at the air-liquid interface. A short range attractive force can be induced by careful choice of buoyancy and capillarity to create self-assembled particle structures which can be tracked by imaging. First, the particles are placed randomly at the interface, and then aggregation is induced by smoothly decreasing the area of the interface which causes the particles to come within the attractive force range caused by capillarity. We measure the area fraction at which the connectivit...
arXiv: Soft Condensed Matter, 2020
Twisting sheets as a strategy to form yarns with nested structure lacks scientific guiding princi... more Twisting sheets as a strategy to form yarns with nested structure lacks scientific guiding principles but relies on millennia of human experience in making catguts, food packaging, and redeployable fabric wearables. We formulate a tensional twist-folding route to making yarns with prescribed folded, scrolled, and encapsulated architectures by remote boundary loading. By harnessing micro-focus x-ray scanning to noninvasively image the fine internal structure, we show that a twisted sheet follows a surprisingly ordered folding transformation as it self-scrolls to form structured yarns. As a sheet is twisted by a half-turn, we find that the elastic sheet spiral accordion folds with star polygon shapes characterized by Schlafli symbols set by the primary instability. A scalable model incorporating dominant stretching modes with origami kinematics explains not only the observed multilayered structure, torque, and energetics, but also the topological transformation into yarns with prescri...
Bulletin of the American Physical Society, 2011
... Authors: Moumita Dasgupta (Clark University). ... We will further discuss the dependence of s... more ... Authors: Moumita Dasgupta (Clark University). ... We will further discuss the dependence of swimming speed on Deborah number and other characteristics of the fluid. To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2011.MAR.Y9.6. ...
BACKGROUND: Exhalation exposure from patients to healthcare workers (HCWs), while using a nasal c... more BACKGROUND: Exhalation exposure from patients to healthcare workers (HCWs), while using a nasal cannula or simple O2 mask used in treating COVID-19 and other respiratory diseases, is a present and future risk. Little is known on exhalation dispersal through these devices, and on mitigating the viral exposure to those in the vicinity. METHODS: Respiration through O2 therapy devices was studied with a supine manikin equipped with a controllable mechanical lung by measuring aerosol density and flow with direct imaging. Dispersal direction and distances were quantified while placing a surgical mask loosely over the devices and contrasted with unmitigated oxygenation device use. Exhalation jets were examined over the entire range of oxygenation rates used in treatment. RESULTS: Exhalation jets travel 0.35 plus/minus 0.02 m upward while wearing a nasal cannula, and 0.29 plus/minus 0.02 m laterally while wearing a simple O2 mask posing significant inhalation risk. Placing a surgical facema...
Bulletin of the American Physical Society, 2012
Submitted for the DFD12 Meeting of The American Physical Society Speed of a Taylor Swimmer in New... more Submitted for the DFD12 Meeting of The American Physical Society Speed of a Taylor Swimmer in Newtonian and Viscoelastic Fluids MOUMITA DASGUPTA, Clark University, BIN LIU, Brown University, HENRY FU, University of Nevada, Reno, MICHAEL BERHANU, MSC Paris Diderot University, KENNETH BREUER, THOMAS POWERS, Brown University, ARSHAD KUDROLLI, Clark University — We demonstrate that the speed of an idealized Taylor swimmer with a prescribed waveform in a viscoelastic fluid can be greater or lesser than in a Newtonian fluid depending on their rheological properties. The measurements are performed using a cylindrical sheet immersed in a fluid inside a cylindrical tank under torque free conditions with traveling waves imposed in the azimuthal direction. Swimming speeds in the Newtonian case are found to be consistent with calculations using the Stokes equation. A faster swimming speed is found in a viscoelastic fluid which has a constant viscosity with shear rate. By contrast, a slower swim...
ABSTRACT Living organisms exhibit remarkable cooperative behavior in response to environmental st... more ABSTRACT Living organisms exhibit remarkable cooperative behavior in response to environmental stress. Bacterial colonies growing on a nutrient-rich substrate have served as model systems for studying pattern formation and population dynamics in biological systems. We report a novel spatio-temporal response of a Bacillus subtilis colony initially growing under ambient conditions to ultraviolet (UV) radiation exposure. The growth of the colonies decreased, but remarkably, the bacteria at the center of the colonies migrated towards the colony edge forming a ring during uniform exposure. The migration of the bacteria has no obvious benefit since they are still under UV radiation of the same strength. When the radiation is switched off, the colonies were observed to grow both inward into the evacuated regions as well as outward indicating that the pattern is not a swarming ring formed due to depletion of nutrients at the center of the colony. We also discuss the results of directed random walk simulations. Using simple models we show the rearrangement is not a consequence of linear attraction or repulsion between the bacteria.
ABSTRACT We study the effect of liquids on the angle of inclination of a wet sandpile in a rotati... more ABSTRACT We study the effect of liquids on the angle of inclination of a wet sandpile in a rotating drum system. In this system, the surface exhibits stick slip motion for slow rotation rates omega, and continuos avalanching above a critical omega. We will focus on the stick-slip regime, where the angle of inclination of the pile oscillates between the maximum angle of stability before an avalanche, and the angle of repose after the avalanche. Both angles are observed to increase and saturate as a function of the volume fraction of the fluid. Furthermore, by changing the viscosity of the fluid using water-glycerol mixtures, we observe that both the maximum angle of stability and the angle of repose increase with the viscosity of the fluid. There are two possible explanations for the increase of the angle of stability of the pile. First, there may be creep motion between the particles giving rise to viscous forces before an avalanche, that are too small to observe visually. The creep motion is also slower than the rate of increase of the surface due to rotation. Second, the average number of liquid bridges between particles may increase with viscosity. We will comment on both possibilities using experimental data and scaling arguments.
Submitted for the MAR06 Meeting of The American Physical Society Curvature condensation and twinn... more Submitted for the MAR06 Meeting of The American Physical Society Curvature condensation and twinning in an indented elastic shell MOUMITA DAS, ASHKAN VAZIRI, Harvard University, ARSHAD KUDROLLI, Clark University, L. MAHADEVAN, Harvard University — We study the formation of a localized geometrical defect and its evolution in an elastic shell using a combination of experiment and numerical simulation. We find that as a symmetric localized indentation on a semi-cylindrical shell increases, there is a transition from a global mode of deformation to a localized one which leads to the condensation of curvature along a parabolic crease along the line of symmetry. Further indentation leads to a twinning phenomena wherein the crease bifurcates into two defects that move apart on either side of the line of symmetry. We present a simple theory to explain the main features of the experiments and numerical simulations. Moumita Das University of California, Los Angeles Date submitted: 21 Nov 2005...
We investigate the erosive growth of channels in a thin subsurface sedimentary layer driven by hy... more We investigate the erosive growth of channels in a thin subsurface sedimentary layer driven by hydrodynamic drag toward understanding subterranean networks and their relation to river networks charged by ground water. Building on a model based on experimental observations of fluid-driven evolution of bed porosity, we focus on the characteristics of the channel growth and their bifurcations in a horizontal rectangular domain subject to various fluid source and sink distributions. We find that the erosion front between low- and high-porosity regions becomes unstable, giving rise to branched channel networks, depending on the spatial fluctuations of the fluid flow near the front and the degree to which the flow is above the erodibility threshold of the medium. Focusing on the growth of a network starting from a single channel, and by identifying the channel heads and their branch points, we find that the number of branches increases sublinearly and is affected by the source distributio...
We investigate mucosalivary dispersal and deposition on horizontal surfaces corresponding to huma... more We investigate mucosalivary dispersal and deposition on horizontal surfaces corresponding to human exhalations with physical experiments under still-air conditions. Synthetic fluorescence tagged sprays with size and speed distributions comparable to human sneezes are observed with high-speed imaging. We show that while some larger droplets follow parabolic trajectories, smaller droplets stay aloft for several seconds and settle slowly with speeds consistent with a buoyant cloud dynamics model. The net deposition distribution is observed to become correspondingly broader as the source height H is increased, ranging from sitting at a table to standing upright. We find that the deposited mucosaliva decays exponentially in front of the source, after peaking at distance x = 0.71 m when H = 0.5 m, and x = 0.56 m when H = 1.5 m, with standard deviations ≈0.5 m. Greater than 99% of the mucosaliva is deposited within x = 2 m, with faster landing times further from the source. We then demonst...
Physical Review Letters
We investigate the development of mobility inversion and fingering when a granular suspension is ... more We investigate the development of mobility inversion and fingering when a granular suspension is injected radially between horizontal parallel plates of a cell filled with a miscible fluid. While the suspension spreads uniformly when the suspension and the displaced fluid densities are exactly matched, even a small density difference is found to result in a dense granular front which develops fingers with angular spacing that increase with granular volume fraction and decrease with injection rate. We show that the time scale over which the instability develops is given by the volume fraction dependent settling time scale of the grains in the cell. We then show that the mobility inversion and the non-equilibrium Korteweg surface tension due to granular volume fraction gradients determine the number of fingers at the onset of the instability in these miscible suspensions.
Proceedings of the National Academy of Sciences
We investigate the dynamics ofLumbriculus variegatusin water-saturated sediment beds to understan... more We investigate the dynamics ofLumbriculus variegatusin water-saturated sediment beds to understand limbless locomotion in the benthic zone found at the bottom of lakes and oceans. These slender aquatic worms are observed to perform elongation–contraction and transverse undulatory strokes in both water-saturated sediments and water. Greater drag anisotropy in the sediment medium is observed to boost the burrowing speed of the worm compared to swimming in water with the same stroke using drag-assisted propulsion. We capture the observed speeds by combining the calculated forms based on resistive-force theory of undulatory motion in viscous fluids and a dynamic anchor model of peristaltic motion in the sediments. Peristalsis is found to be effective for burrowing in noncohesive sediments which fill in rapidly behind the moving body inside the sediment bed. Whereas the undulatory stroke is found to be effective in water and in shallow sediment layers where anchoring is not possible to a...
Physical Review E
We measure the drag encountered by a vertically oriented rod moving across a sedimented granular ... more We measure the drag encountered by a vertically oriented rod moving across a sedimented granular bed immersed in a fluid under steady-state conditions. At low rod speeds, the presence of the fluid leads to a lower drag because of buoyancy, whereas a significantly higher drag is observed with increasing speeds. The drag as a function of depth is observed to decrease from being quadratic at low speeds to appearing more linear at higher speeds. By scaling the drag with the average weight of the grains acting on the rod, we obtain the effective friction µe encountered over six orders of magnitude of speeds. While a constant µe is found when the grain size, rod depth and fluid viscosity are varied at low speeds, a systematic increase is observed as the speed is increased. We analyze µe in terms of the inertial number I and viscous number J to understand the relative importance of inertia and viscous forces, respectively. For sufficiently large fluid viscosities, we find that the effect of varying the speed, depth, and viscosity can be described by the empirical function µe = µo + kJ n , where µo is the effective friction measured in the quasi-static limit, and k and n are material constants. The drag is then analyzed in terms of the effective viscosity ηe and found to decrease systematically as a function of J. We further show that ηe as a function of J is directly proportional to the fluid viscosity and the µe encountered by the rod.
Physical Review Fluids
We investigate the effective friction encountered by an intruder moving through a sedimented medi... more We investigate the effective friction encountered by an intruder moving through a sedimented medium which consists of transparent granular hydrogels immersed in water, and the resulting motion of the medium. We show that the effective friction µ e on a spherical intruder is captured by the inertial number I given by the ratio of the time scale over which the intruder moves and the inertial time scale of the granular medium set by the overburden pressure. Further, µ e is described by the function µ e (I) = µ s + αI β , where µ s is the static friction, and α and β are material dependent constants which are independent of intruder depth and size. By measuring
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science
We investigate with experiments the twist-induced transverse buckling instabilities of an elastic... more We investigate with experiments the twist-induced transverse buckling instabilities of an elastic sheet of length L , width W and thickness t , that is clamped at two opposite ends while held under a tension T . Above a critical tension T λ and critical twist angle η tr , we find that the sheet buckles with a mode number n ≥1 transverse to the axis of twist. Three distinct buckling regimes characterized as clamp-dominated, bendable and stiff are identified, by introducing a bendability length L B and a clamp length L C (< L B ). In the stiff regime ( L > L B ), we find that mode n =1 develops above η tr ≡ η S ∼( t / W ) T −1/2 , independent of L . In the bendable regime L C < L < L B , n =1 as well as n >1 occur above η tr ≡ η B ∼ t / L T − 1 / 4 . Here, we find the wavelength λ B ∼ L t T − 1 / 4 , when n >1. These scalings agree with those derived from a covariant form of the Föppl-von Kármán equations, however, we find that the n =1 mode also occurs over a surpri...
Physical Review E
We investigate the deformation of a longitudinally stretched rectangular sheet which is clamped a... more We investigate the deformation of a longitudinally stretched rectangular sheet which is clamped at two opposite boundaries and free otherwise with experiments, numerical analysis and asymptotic analysis of the biharmonic elastic equation governing their planar equilibrium configurations. The displacement field of the sheet is measured by tracking embedded fluorescent tracers with a digital image correlation (DIC) technique. The experiments and numerical finite element analysis (FEA) are found to be in overall good agreement except at the very corners where large deformations occur. We find that the deformed sheet can be broadly divided into a uniaxially stretched central region and two clamp dominated side regions. A subregion characterized by a diverging stress can be identified at each of the four clamped-free corners within the clamp dominated region. We postulate that the divergence at the corners is regularized by nonlinear elastic deformations occurring in this subregion at the very corners and provide a nontrivial scaling for its size. Within the intermediate corner dominated subregion, measured displacements grow with distance r from the corners as r α , with power α < 1 consistent with the development of stress singularities at the intersection of the free and clamped edges.