Niels Bohr Lecture by Christian Wagner, Saarland University

The Physics of Blood Flow: From Soft Matter Science to Clinical Research

ABSTRACT

Blood is a complex fluid in which soft, deformable cells interact with flow, confinement, and each other. I will begin with a brief general introduction to complex fluids, and then use blood flow as an example of how basic physical concepts can help us understand processes ranging from the cellular scale to clinical diagnostics. At the same time, biological systems such as blood reveal physical phenomena that go beyond what is typically observed in classical synthetic colloidal systems.

I will discuss examples from in vitro, in vivo, and in silico studies that illustrate how red blood cells organize, deform, and interact in different situations. At rest, their aggregation gives rise to the erythrocyte sedimentation rate, a classical clinical inflammation test that can be understood as the sedimentation of a fragile gel formed by very soft objects. Using holographic optical tweezers, we determine the interaction energies and mechanisms between individual cells and study the collapse of the resulting gel with light-sheet microscopy. Counterintuitively, this gel becomes more fragile when the cell–cell interaction energy is increased, providing a simple example of how soft-matter physics can change the interpretation of a routine diagnostic test.

In flow, red blood cells can pass through capillaries narrower than their own diameter because of their remarkable deformability, giving rise to pronounced fluid–structure interactions and rich collective behaviour. In controlled microfluidic experiments, such flows reveal shape transitions that provide access to mechanical properties such as the membrane shear modulus and the cytosol viscosity. The final part of the talk will then move to in vivo studies in rodents, where blood flow can be followed in capillary networks at the level of individual cells. These measurements show how red and white blood cells are redistributed at bifurcations and how predictions from lab-on-a-chip experiments and theoretical models are modified by the complexity of living tissue.

BIOGRAPHY

Christian Wagner is Professor of Experimental Physics at Saarland University, where he leads research on experimental statistical physics, complex fluids, and biophysics with a particular focus on red blood cell mechanics and microcirculation. After completing his PhD at Saarland University in 2000, he held a Marie Curie fellowship at the École Normale Supérieure in Paris and worked in industrial research and development at Agilent Technologies in Karlsruhe on lab-on-a-chip technology before returning to Saarland University, where he became Junior Professor in 2003 and Full Professor in 2007. He also holds a permanent visiting professorship at the University of Luxembourg. He is the spokesperson of several interdisciplinary and international research initiatives, including the DFG Research Unit FOR 2688 “Instabilities and Bifurcations in Pulsatile Flows” and the German-French-Moroccan doctoral programme “Living Fluids”. His work has been recognised by several awards including the Ars legendi Faculty Award for Mathematics and Natural Sciences for best teaching. In addition to his research and teaching, he has held several academic leadership positions, including Vice President for Planning and Strategy and Chief Digital Officer of Saarland University, and is responsible for Saarland University’s trinational physics degree programme.