Tom Verwijlen

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.

Separating viscoelastic and compressibility contributions in pressure-area isotherm measurements

Monolayers of surface active molecules or particles play an important role in biological systems as well as in consumer products. Their properties are controlled by thermodynamics as well as the mechanical properties of the interface itself. For insoluble species forming Langmuir monolayers, surface pressure-area isotherms are typically used to characterize the thermodynamic state. A Langmuir trough equipped with a Wilhelmy plate is often used for such measurements. However, when Langmuir interfaces are compressed and become more structured, the elastic response of these interfaces can interfere with the measurement of the surface pressure-area isotherm, even when the compression speed is slow. Recent reports of compression data for highly elastic interfaces revealed a dependence of the apparent surface pressures on the geometry of the measurement trough. In the present work, this dependence is investigated by considering adequate constitutive models. Since deformations in such compression experiments can be large, linearized versions of the Kelvin-Voigt model do not suffice. We develop a framework for quasi-linear constitutive models by choosing suitable non-linear strain tensors, adequately separating the shear and dilatational effects in a frame invariant manner. The proposed constitutive models can be used as building blocks to describe viscoelastic behavior as well. The geometry dependence in isotherm measurements is then shown to be a consequence of varying contributions of the isotropic surface pressure and extra shear and dilatational elastic stresses. Using these insights, an approach is proposed to obtain the intrinsic surface pressure-area isotherms for elastic interfaces. As a case study, experimental data on graphene oxidesheets at the air-water interface is investigated to evaluate the proposed model.

Soft-Glassy Rheology of Asphaltenes at Liquid Interfaces

The presence of asphaltenes in oil and water emulsions promotes emulsion stability, a major concern in the recovery of oil reserves. A number of mechanisms for this stabilizing effect have been proposed recently, including those that implicate the interfacial rheology of asphaltenes as a primary factor. We present interfacial shear rheological data for asphaltenes at liquid–liquid interfaces and show conclusively that asphaltenes at an oil–water interface exhibit a soft-glassy rheological behavior in agreement with the soft-glassy rheology (SGR) model. This characteristic allows the stabilization of emulsions via mechanical resistance to coalescence. GRAPHICAL ABSTRACT

Extensional rheometry at interfaces: Analysis of the Cambridge Interfacial Tensiometer

Whereas devices for measuring the interfacial shear and dilatational rheology are readily available, extensional rheometry at interfaces remains essentially unexplored. However, a setup mimicking a 2D filament stretching rheometer, the Cambridge Interfacial Tensiometer, was proposed for this very purpose [Jones and Middelberg, Chem. Eng. Sci. 57, 1711–1722 (2002)]. In the present work, a framework is presented for analyzing the interfacial flow field in such device for Newtonian interfaces in the presence of Marangoni flows. Based on the dimensionless numbers that govern the interfacial flow field, different dominant flow types can be identified and the sensitivity of the device for measuring the extensional interfacial viscosity is determined. For the flow field to be dominated by extensional deformations, either the Marangoni number or the ratio of dilatational viscosity to shear viscosity should be at least an order of magnitude higher than the Trouton ratio. Using an analysis for Newtonian materials, ...

Simple microfluidic stagnation point flow geometries

A geometrically simple flow cell is proposed to generate different types of stagnation flows, using a separation flow and small variations of the geometric parameters. Flows with high local deformation rates can be changed from purely rotational, over simple shear flow, to extensional flow in a region surrounding a stagnation point. Computational fluid dynamic calculations are used to analyse how variations of the geometrical parameters affect the flow field. These numerical calculations are compared to the experimentally obtained streamlines of different designs, which have been determined by high speed confocal microscopy. As the flow type is dictated predominantly by the geometrical parameters, such simple separating flow devices may alleviate the requirements for flow control, while offering good stability for a wide variety of flow types.