Travis W. Walker

Microvascular Endothelial Cells Migrate Upstream and Align Against the Shear Stress Field Created by Impinging Flow

At present, little is known about how endothelial cells respond to spatial variations in fluid shear stress such as those that occur locally during embryonic development, at heart valve leaflets, and at sites of aneurysm formation. We built an impinging flow device that exposes endothelial cells to gradients of shear stress. Using this device, we investigated the response of microvascular endothelial cells to shear-stress gradients that ranged from 0 to a peak shear stress of 9-210 dyn/cm(2). We observe that at high confluency, these cells migrate against the direction of fluid flow and concentrate in the region of maximum wall shear stress, whereas low-density microvascular endothelial cells that lack cell-cell contacts migrate in the flow direction. In addition, the cells align parallel to the flow at low wall shear stresses but orient perpendicularly to the flow direction above a critical threshold in local wall shear stress. Our observations suggest that endothelial cells are exquisitely sensitive to both magnitude and spatial gradients in wall shear stress. The impinging flow device provides a, to our knowledge, novel means to study endothelial cell migration and polarization in response to gradients in physical forces such as wall shear stress.

Insertion mechanism of a poly(ethylene oxide)-poly(butylene oxide) block copolymer into a DPPC monolayer.

Interactions between amphiphilic block copolymers and lipids are of medical interest for applications such as drug delivery and the restoration of damaged cell membranes. A series of monodisperse poly(ethylene oxide)-poly(butylene oxide) (EOBO) block copolymers were obtained with two ratios of hydrophilic/hydrophobic block lengths. We have explored the surface activity of EOBO at a clean interface and under 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayers as a simple cell membrane model. At the same subphase concentration, EOBO achieved higher equilibrium surface pressures under DPPC compared to a bare interface, and the surface activity was improved with longer poly(butylene oxide) blocks. Further investigation of the DPPC/EOBO monolayers showed that combined films exhibited similar surface rheology compared to pure DPPC at the same surface pressures. DPPC/EOBO phase separation was observed in fluorescently doped monolayers, and within the liquid-expanded liquid-condensed coexistence region for DPPC, EOBO did not drastically alter the liquid-condensed domain shapes. Grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XRR) quantitatively confirmed that the lattice spacings and tilt of DPPC in lipid-rich regions of the monolayer were nearly equivalent to those of a pure DPPC monolayer at the same surface pressures.

Role of fluid elasticity on the dynamics of rinsing flow by an impinging jet

Rinsing flows are common processes where a jet of one liquid impinges upon a layer of a second liquid for the purpose of removing the second liquid. An imaging setup has been developed to obtain both qualitative and quantitative data on the rinsing flow of a jet of water impinging on either layers of Newtonian or elastic fluids. Three classes of test fluids have been investigated: a Newtonian glycerol-water solution, a semidilute aqueous solution of high molecular weight polyacrylamide solution displaying both elasticity and shear thinning, and an elastic but non-shear thinning Boger fluid. The fluids were designed to have approximately equal zero-shear viscosities. For all cases, a circular hydraulic jump occurs and Saffman–Taylor instabilities were observed at the interface between the low viscosity jet and the higher viscosity coating liquids. Results show that the elasticity (extensional viscosity) of the samples influences the pattern of the instabilities and contributes to dampening surface disturba...

Scaling analysis and mathematical theory of the interfacial stress rheometer

The interfacial stress rheometer (ISR), uses the oscillations of a magnetic needle suspended on an interface to characterize the dynamic moduli of thin films. Mathematical theories to interpret the device have developed slowly because of the strong coupling between the stresses in the surface and the bulk subphase. In this work, we simplify the equations of motion by introducing new length scales and reinterpreting the dimensionless numbers. Several Greens functions are developed for typical ISR geometries, leading to a set of boundary element methods for the full numerical solution of the equations of motion. Using Taylor series, a multipole expansion is extracted from the boundary integral equations, and we show that both numerical methods converge in under five elements. Analytical theories are developed for the cases of small and large interfacial stress, proving that the finite size of the needle has an O(1) effect and reinforcing the physics behind the length scales and dimensionless groupings. We ...

Nanoscale Patterning of Extracellular Matrix Alters Endothelial Function under Shear Stress

The role of nanotopographical extracellular matrix (ECM) cues in vascular endothelial cell (EC) organization and function is not well-understood, despite the composition of nano- to microscale fibrillar ECMs within blood vessels. Instead, the predominant modulator of EC organization and function is traditionally thought to be hemodynamic shear stress, in which uniform shear stress induces parallel-alignment of ECs with anti-inflammatory function, whereas disturbed flow induces a disorganized configuration with pro-inflammatory function. Since shear stress acts on ECs by applying a mechanical force concomitant with inducing spatial patterning of the cells, we sought to decouple the effects of shear stress using parallel-aligned nanofibrillar collagen films that induce parallel EC alignment prior to stimulation with disturbed flow resulting from spatial wall shear stress gradients. Using real time live-cell imaging, we tracked the alignment, migration trajectories, proliferation, and anti-inflammatory behavior of ECs when they were cultured on parallel-aligned or randomly oriented nanofibrillar films. Intriguingly, ECs cultured on aligned nanofibrillar films remained well-aligned and migrated predominantly along the direction of aligned nanofibrils, despite exposure to shear stress orthogonal to the direction of the aligned nanofibrils. Furthermore, in stark contrast to ECs cultured on randomly oriented films, ECs on aligned nanofibrillar films exposed to disturbed flow had significantly reduced inflammation and proliferation, while maintaining intact intercellular junctions. This work reveals fundamental insights into the importance of nanoscale ECM interactions in the maintenance of endothelial function. Importantly, it provides new insight into how ECs respond to opposing cues derived from nanotopography and mechanical shear force and has strong implications in the design of polymeric conduits and bioengineered tissues.

Role of shear-thinning on the dynamics of rinsing flow by an impinging jet

Using a jet of one fluid to ablate a second liquid that coats a planar substrate produces a variety of interesting flow structures. These rinsing flows can create alternating layers of jet and coating fluids, creating difficulties in imaging the resulting radial hydraulic jumps for qualitative and quantitative data extraction. This study is an extension of the work done by Hsu et al. [“Role of fluid elasticity on the dynamics of rinsing flow by an impinging jet,” Phys. Fluids. 23, 033101 (2011)10.1063/1.3567215] and presents a method to reveal the positions of the free surfaces in rinsing flows as well as exploring the role of shear thinning in this flow process. Following the previous work, we used an impinging jet of water to rinse coating fluids of varying rheologies to understand the flow structures during the transient growth of the resulting hydraulic jumps, observing rheological dependencies on the jump magnitude and velocity and overall topography. While many instabilities have been shown to arise...

Planar Alignment of Magnetic Microdisks in Composites Using Rotating Fields

Soft magnetic composites with aligned anisotropic magnetic particles can have properties not achievable in traditional single-phase materials. These materials are promising for high-frequency inductor and antenna applications. Composites with uniaxial anisotropy, achieved by aligning rod-shaped particles in a constant magnetic field, have previously been reported. In this paper, we discuss the advantages and conditions for realizing composites with planar anisotropy, by aligning disk-shaped particles in a rotating magnetic field. We use Ni-Fe microdisks in a UV-curable matrix as the study system, and present an experimental and theoretical investigation of their alignment under a rotating field in a viscous fluid. The theoretical model is based on Stokes flow of an isolated magnetic oblate ellipsoidal particle in a rotating magnetic field and enables an understanding of the hydrodynamics of the microdisk alignment process. Alignment rate under varying aligning field strength, field rotation frequency, and fluid viscosity is observed using optical microscopy, and characterized by the magnetic properties resultant in the composite. Good agreement is found between measured results and the model. Our work demonstrates for the first time that planar anisotropy in a magnetic composite can be tuned by precise control of aligning field strength and duration, and provides the insight necessary to engineer the alignment dynamics for achieving high-frequency operation.

Planar Alignment of Isolated Magnetic Disks in Newtonian Fluids by a Rotating Field

Magnetic anisotropy can be induced in soft magnetic composites by aligning constituent anisotropic magnetic particles. While the uniaxial alignment by a constant field has been extensively investigated, reported studies on alignment of magnetic particles in a rotating field lack experimental validation. In this effort, we present a systematic experimental and theoretical investigation of the alignment dynamics of isolated disk-shaped magnetic particles in a planar rotating magnetic field in order to fabricate composites with planar anisotropy. The theoretical model is developed assuming: 1) quiescent flow field, 2) low-Reynolds-number Stokes flow, 3) negligible Brownian motion, and 4) oblate-ellipsoid approximation for the magnetic disks. The model is experimentally verified by optical microscopy of disk suspensions for varied rotating field strength and frequency, fluid viscosity, and disk size. Good agreement is obtained between the predicted and measured results, yielding insight to parameters important for the control of anisotropy of composite magnetic materials.

Theoretical study of alignment dynamics of magnetic oblate spheroids in rotating magnetic fields

Magnetic composites containing anisotropic magnetic particles can achieve properties not possible in corresponding bulk or thin films of the magnetic material. In this work, we discuss how planar magnetic anisotropy may be achieved in a composite by aligning disk-shaped particles in an in-plane rotating magnetic field. Previous efforts have reported a simple model of aligning particles in a high-frequency rotating magnetic field. However, no complete analytic solution was proposed. Here, we provide a full analytic solution that describes the alignment dynamics of microdisks in a rotating field that covers the entire frequency range. We also provide simplified solutions at both high-frequency and low-frequency limits through asymptotic expansions for easy implementation into industrial settings. The analytic solution is confirmed by numerical simulation and shows agreement with experiments.

Enhanced particle removal using viscoelastic fluids

The introduction and subsequent removal of highly elastic solutions from surfaces has recently allowed industry to effectively remove colloidal, particulate contaminants from high-grade silicon. The substrate is first coated with the polymeric cleaning solution, and then the solution is removed either by simply rinsing the surface using an impinging water jet or by siphoning the cleaning fluid from the surface. The advantage of this continuous process over conventional techniques is the noninvasive removal while generating limited nonhazardous aqueous waste. Our group investigated the use of polymeric liquids that effectively eliminate particles without damaging the delicate surfaces. To investigate this removal, we studied two different flow types (siphoning and rinsing) of various rheological fluids to understand the governing physics that allow for removal. In this publication, we show that the presence of local shear flows of a viscoelastic fluid having a large elongational viscosity can create polyme...