Papers by Kerstin Nordstrom
Physical review letters, Jan 6, 2014
We study the impact of a projectile onto a bed of 3 mm grains immersed in an index-matched fluid.... more We study the impact of a projectile onto a bed of 3 mm grains immersed in an index-matched fluid. We vary the amount of prestrain on the sample, strengthening the force chains within the system. We find this affects only the prefactor of the linear depth-dependent term in the stopping force. We propose a simple model to account for the strain dependence of this term, owing to increased pressure in the pile. Interestingly, we find that the presence of the fluid does not affect the impact dynamics, suggesting that dynamic friction is not a factor. Using a laser sheet scanning technique to visualize internal grain motion, we measure the trajectory of each grain throughout an impact. Microscopically, our results indicate that weaker initial force chains result in more irreversible, plastic rearrangements, suggesting static friction between grains does play a substantial role in the energy dissipation.
Quantifying Collective Cell Migration during Cancer Progression
A Microstructural View of Burrowing with Roboclam
Rheology of Soft Colloids Near Rigidity Onset: Critical Scaling, Thermal and Non-thermal Responses
Does the Fluid Matter? Impact Into Wet Granular Materials
Soft Matter, 2014
We study the rheological behavior of colloidal suspensions composed of soft sub-micron-size hydro... more We study the rheological behavior of colloidal suspensions composed of soft sub-micron-size hydrogel particles across the liquid-solid transition. The measured stress and strain-rate data, when normalized by thermal stress and time scales, suggest our systems reside in a regime wherein thermal effects are important. In a different vein, critical point scaling predictions for the jamming transition, typical in athermal systems, are tested. Near dynamic arrest, the suspensions exhibit scaling exponents similar to those reported in Nordstrom et al., Phys. Rev. Lett., 2010, 105, 175701. The observation suggests that our system exhibits a glass transition near the onset of rigidity, but it also exhibits a jamming-like scaling further from the transition point. These observations are thought-provoking in light of recent theoretical and simulation findings, which show that suspension rheology across the full range of microgel particle experiments can exhibit both thermal and athermal mechanisms.
Physical Review Letters, 2010
The rheology near jamming of a suspension of soft colloidal spheres is studied using a custom mic... more The rheology near jamming of a suspension of soft colloidal spheres is studied using a custom microfluidic rheometer that provides stress versus strain rate over many decades. We find non-Newtonian behavior below the jamming concentration and yield stress behavior above it. The data may be collapsed onto two branches with critical scaling exponents that agree with expectations based on Hertzian contacts and viscous drag. These results support the conclusion that jamming is similar to a critical phase transition, but with interaction-dependent exponents.
Publisher’s Note: Granular Convection in Microgravity [Phys. Rev. Lett. <span xmlns:xlink="http://www.w3.org/1999/xlink" style="font-weight: bold;">110</span>, 018307 (2013)]
Physical Review Letters, 2013
Physical Review Letters, 2013
We investigate the role of gravity on convection in a dense granular shear flow. Using a microgra... more We investigate the role of gravity on convection in a dense granular shear flow. Using a microgravity-modified Taylor-Couette shear cell under the conditions of parabolic flight microgravity, we demonstrate experimentally that secondary, convective-like flows in a sheared granular material are close to zero in microgravity, and enhanced under high gravity conditions, though the primary flow fields are unaffected by gravity. We suggest that gravity tunes the frictional particle-particle and particle-wall interactions, which have been proposed to drive the secondary flow. In addition, the degree of plastic deformation increases with increasing gravitational forces, supporting the notion that friction is the ultimate cause.
Physical Review E, 2010
An exact method is developed for computing the height of an elastic medium subjected to centrifug... more An exact method is developed for computing the height of an elastic medium subjected to centrifugal compression, for arbitrary constitutive relation between stress and strain. Example solutions are obtained for power-law media and for cases where the stress diverges at a critical strain -for example as required by packings composed of deformable but incompressible particles. Experimental data are presented for the centrifugal compression of thermo-responsive N-isopropylacrylamide (NIPA) microgel beads in water. For small radial acceleration, the results are consistent with Hertzian elasticity, and are analyzed in terms of the Young elastic modulus of the bead material. For large radial acceleration, the sample compression asymptotes to a value corresponding to a space-filling particle volume fraction of unity. Therefore we conclude that the gel beads are incompressible, and deform without deswelling. In addition, we find that the Young elastic modulus of the particulate gel material scales with cross-link density raised to the power 3.3 ± 0.8, somewhat larger than the Flory expectation.
Physical Review E, 2011
We present experimental measurements of dynamical heterogeneities in a dense system of microgel s... more We present experimental measurements of dynamical heterogeneities in a dense system of microgel spheres, sheared at different rates and at different packing fractions in a microfluidic channel, and visualized with high speed digital video microscopy. A four-point dynamic susceptibility is deduced from video correlations, and is found to exhibit a peak that grows in height and shifts to longer times as the jamming transition is approached from two different directions. In particular, the time for particle-size root-mean square relative displacements is found to scale as τ * ∼ (γ∆φ 4 ) −1 whereγ is the strain rate and ∆φ = |φ − φc| is the distance from the random close packing volume fraction. The typical number of particles in a dynamical heterogeneity is deduced from the susceptibility peak height and found to scale as n * ∼ (γ∆φ 4 ) −0.3 . Exponent uncertainties are less than ten percent. We emphasize that the same power-law behavior is found at packing fractions above and below φc. Thus, our results considerably extend a previous observation of n * ∼γ −0.3 for granular heap flow at fixed packing below φc. Furthermore, the implied result n * ∼ (τ * ) 0.3 compares well with expectation from mode-coupling theory and with prior observations for driven granular systems.
New Journal of Physics, 2013
Although understanding the collective migration of cells, such as that seen in epithelial sheets,... more Although understanding the collective migration of cells, such as that seen in epithelial sheets, is essential for understanding diseases such as metastatic cancer, this motion is not yet as well characterized as individual cell migration. Here we adapt quantitative metrics used to characterize the flow and deformation of soft matter to contrast different types of motion within a migrating sheet of cells. Using a finite-time Lyapunov exponent (FTLE) analysis, we find that-in spite of large fluctuations-the flow field of an epithelial cell sheet is not chaotic. Stretching of a sheet of cells (i.e. positive FTLE) is localized at the leading edge of migration and increases when the cells are more highly stimulated. By decomposing the motion of the cells into affine and non-affine components using the metric D 2 min , we quantify local plastic rearrangements and describe the motion of a group of cells in a novel way. We find an increase in plastic rearrangements with increasing cell densities, whereas inanimate systems tend to exhibit less non-affine rearrangements with increasing density.
Icarus, 2012
The authors regret that in the original publishing of this article, was incorrect. The correct fi... more The authors regret that in the original publishing of this article, was incorrect. The correct figure and legend are now produced below. Fig. . The degree of local order (i.e., packing density) at the position of each particle in a numerical simulation when the small particle concentration is 10%. Grain boundary (GB) regions determined using the algorithm of Berardi et al. (2010). Black signifies near-hexagonal particle packing with w 6 close to 1. Grey and white correspond to more disordered packing with w 6 < 0.7 (i.e., GB regions). See Eq. (1) for the definition of w 6 . The locations of the small (2 mm) particles are all marked with an Â. The laboratory experiment results are not shown here, but in both the experiments and the numerical simulations the small particles are almost all located in grain boundaries. Particles are not drawn to scale.
Icarus, 2012
Surfaces of planets and small bodies of our Solar System are often covered by a layer of granular... more Surfaces of planets and small bodies of our Solar System are often covered by a layer of granular material that can range from a fine regolith to a gravel-like structure of varying depths. Therefore, the dynamics of granular materials are involved in many events occurring during planetary and small-body evolution thus contributing to their geological properties.
Biophysical Journal, 2011
in regulation of cell division, membrane stability and matrix-elasticity sensing during human hem... more in regulation of cell division, membrane stability and matrix-elasticity sensing during human hematopoietic stem cell (HSC) differentiation. Myosin is required for the survival of proliferating myeloid progenitors, while long-term primitive HSCs are resistant to myosin inhibition. In contrast, inhibition of cellular contractility facilitates megakaryocyte (MK) maturation by polyploidization and fragmentation into functional platelets in vivo (median fragmentation threshold at~1mN/m in vitro). In addition, differences in local tissue stiffnesssuch as exists between cortical bone and marrow -likely contribute to HSC differentiation because in vitro adhesion to stiff (34kPa) collagenous matrices tends to inhibit MK maturation whenever myosin is inhibited, while soft (0.3kPa) matrices with lower amount of collagens tend to facilitate this process. Quantitative mass spectrometry and confocal microscopy indicate both global remodeling of cytoskeletal proteomes and extensive lamin network formation during MK maturation, providing MKs in bone marrow with an ideal structure to shed platelets into permeating capillaries. Together, these data show that cell relaxation and soft matrices maintain primitiveness of HSCs and drive MK differentiation, whereas the opposite physical cues support HSC differentiation into myeloid progenitors.
Biophysical Journal, 2009
Cell types from many tissues respond to changes in substrate stiffness by actively remodeling the... more Cell types from many tissues respond to changes in substrate stiffness by actively remodeling their cytoskeletons to alter spread area or adhesion strength, and in some cases changing their own stiffness to match that of their substrate. These cell responses to substrate stiffness are linked to substrate-induced changes in the state, localization, and amount of numerous proteins, but detailed evidence for the requirement of specific proteins in these distinct forms of mechanical response are scarce. Here we use microfluidics techniques to produce gels with a gradient of stiffness to show the essential function of filamin A in cell responses to mechanical stimuli and dissociate cell spreading and stiffening by contrasting responses of a pair of human melanoma-derived cell lines that differ in expression of this actin cross-linking protein. M2 melanoma cells null for filamin A do not alter their adherent area in response to increased substrate stiffness when they link to the substrate only through collagen receptors, but change adherent area normally when bound through fibronectin receptors. In contrast, filamin A-replete A7 cells change adherent area on both substrates and respond more strongly to collagen I-coated gels than to fibronectin-coated gels. Strikingly, A7 cells alter their stiffness, as measured by atomic force microscopy, to match the elastic modulus of the substrate immediately adjacent to them on the gradient. M2 cells, in contrast, maintain a constant stiffness on all substrates that is as low as that of A7 cells on the softest gels examined (1000 Pa). Comparison of cell spreading and cell stiffening on the same gradient substrates shows that cell spreading is uncoupled from stiffening. At saturating collagen and fibronectin concentrations, adhesion of M2 cells is reduced compared to that of A7 cells to an extent approximately equal to the difference in adherent area. Filamin A appears to be essential for cell stiffening on collagen, but not for cell spreading on fibronectin. These results have implications for different models of cell protrusion and adhesion and identify a key role for filamin A in altering cellular stiffness that cannot be compensated for by other actin cross-linkers in vivo.
Microfluidic Rheology of Soft Colloidal Suspensions
ABSTRACT
Rheology of Soft Suspensions near Jamming
ABSTRACT
Microfluidic Rheology of Soft Colloids near Jamming
ABSTRACT
Impact Dynamics in a 3D Granular Bed
ABSTRACT Granular impacts have been studied for many years in the scientific community. Force law... more ABSTRACT Granular impacts have been studied for many years in the scientific community. Force laws have been characterized for different systems including universal scaling relations by Katsuragi and Durian. Despite this, little is known regarding the microscopic origin of these observations. In this work, we study the impact of a projectile onto a bed of 3 mm grains immersed in an index-matched fluid. Using a laser sheet scanning technique, a high speed camera, and particle tracking, we can measure the trajectory of each grain throughout an impact event. We have characterized the nonaffine motion within the system as a function of projectile shape and initial sample preparation. Our preliminary results show significant nonaffine motion near the impactor. These results are compared and contrasted with recent experiments and simulations involving 2D systems.
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Papers by Kerstin Nordstrom