Jan Dhont is a Director at Forschungszentrum Jülich (Institute of Complex Systems) and a full Professor at the Physics Department at the Heinrich-Heine University in Düsseldorf. After his PhD in Physical Chemistry at the van’t Hoff Laboratory at the University of Utrecht and some years as a postdoc in Theoretical Physics at the University of Konstanz he returned to Utrecht as an Associate Professor before he accepted his current position at FZ Jülich. His research interests include the dynamics of macromolecules, the response of several types of macromolecular systems to external fields, and field-induced instabilities in (bio-)colloidal and polymeric systems. Professor Dhont has an impressive track record (> 175 peer-reviewed articles and book chapters; h-Index = 39) and has attained an international high profile in the Soft Matter community due to his important contributions both as an experimentalist as well as a theoretician.

Venue: LINXS Workshop room, Ideon Delta 5, Scheelevägen 19, 5th floor (map)

Time: 14.15 - 15.00 (**Coffee and cake is served after the seminar**)

Title: *Non-uniformly Flowing Suspensions - Non-local stresses, shear-gradient induced mass transport, and a shear-banding instability in systems with a yield stress *

Non-local stresses in colloidal suspensions arise when the shear rate significantly changes over length scales of the order of the size of the colloids. Such stresses are important, for example, in micro-fluidics devices and in systems that exhibit shear-banding instabilities. There is so far no theory, nor simulations, where constitutive relations are derived or simulated that account for such non-local stresses. In the first part of this presentation a constitutive relation is derived that includes non-local stresses, which are characterized by the so-called “shear-curvature viscosity”. An explicit expression for the shear-curvature viscosity is derived, which is validated by Brownian Dynamics simulations [1].

In the second part of this presentation, mass transport due to spatial gradients in the shear rate is discussed. Such mass transport plays an important role in, for example, blood flow (see the figure). An expression is derived for the shear-rate dependence of Fick’s diffusion coefficient (which quantifies shear-induced diffusion) and for the transport coefficient that describes shear-gradient induced mass transport, the so-called “shear-gradient coefficient” [2].

In the third part of this presentation, a shear-banding instability is addressed which results from shear-gradient induced mass transport. The advection-diffusion equation that includes shear-gradient induced mass transport, and a constitutive equation that includes non-local stresses, are solved numerically for a Couette geometry [2]. It is essential to include non-local stresses in the constitutive equation in order to stabilize the system against arbitrary fast growth of arbitrary large gradients in the flow velocity. The resulting stability diagram and shear-banded flow profiles will be compared to experiments on hard-sphere colloidal glasses [4].

Finally, the various types of inhomogeneous flows as observed in Wigner glasses of rod-like colloids will be discussed [5]. The experimental light-scattering method used to probe such non-uniform profiles might be extended to much higher spatial resolution by using X-rays instead of visible light. This might be useful for the future study of flows in micro-fluidics devices.

[1] H. Jin, K. Kang, K.H. Ahn, W.J. Briels, J.K.G. Dhont, *Non-local stresses in highly non-uniformly flowing suspensions: The shear-curvature viscosity, *J. Chem. Phys. 149 (2018) 014903.

[2] H. Jin, K. Kang, H.K. Ahn, J.K.G. Dhont, *Flow instability due to coupling of shear-gradients with concentration: non-uniform flow of (hard-sphere) glasses, *Soft Matter 10 (2014) 9470.

[3] D. Katanov, G. Gompper, D.A. Fedosov, *Microvascular blood flow resistance: Role of red blood cell migration and dispersion, *Microvascular Research, 99 (2015) 57.

[4] R. Besseling, L. Isa, P. Ballesta, G. Petekidis, M. E. Cates, and W. C. K. Poon, *Shear banding and flow-concentration coupling in colloidal glasses, *Phys. Rev. Lett. 105 (2010) 268301.

[5] J. K. G. Dhont, K. Kang, H. Kriegs, O. Danko, J. Marakis, Vlassopoulos,* Nonuniform flow in soft glasses of colloidal rods*, Phys. Rev. Fluids 2 (2017) 043301.