PhD Discussions Sessions: Ignasi Jorba and Jaideep Katuri

Multiscale Nonlinear Mechanics of Lung Extracellular Matrix

A precise knowledge of the mechanical properties of the extracellular matrix (ECM) is critical to further our understanding of the cell-matrix interplay. Atomic force microscopy (AFM) is particularly suitable to study the mechanical properties of ECM at the microscale that cells sense the stiffness of their microenvironment. Nevertheless, although many biological tissues including those of heart and lung are physiologically subjected to stretch, conventional AFM systems do not allow measurement of the stiffness of the sample at different stretch levels. We studied nonlinear micromechanical properties of lung ECM by means of AFM using a novel device fabricated with a 3D printer to stretch the ECM sample during AFM measurements. To compare micro and macroscale mechanics we also probed ECM by means of tensile tests. Finally, we are developing a mathematical model to explain the mechanical behavior at the microscale and macroscale and the relation between them.

Chemically active Janus colloids near surfaces

Self-propelled colloidal particles have recently emerged as an important class of active matter. A prime example is ‘Janus’ particles which have heterogenous catalytic properties along their surface and self-generate local chemical gradients in the presence of a fuel.1 During propulsion, these particles create hydrodynamic and phoretic fields around them, via which they interact with their local environment and nearby particles.2 In this talk I will begin with describing experiments where we exploit these hydrodynamic and phoretic interactions with nearby surfaces to develop a robust guidance system for these micron sized objects. We find that the presence of nearby surfaces induces a strong alignment interaction that can be used to ensure that self-propelled particles follow along pre-defined pathways. We also show that this effect is dependent on the rate of chemical activity and probe the limits of topographical properties below which this mechanism fails.3 In the second part of the talk, I will describe a directional confinement that naturally emerges for spherical, chemically active Janus colloids in channel flow. We find that the interplay between chemical activity, confinement near a surface and imposed flow can lead to a strong attraction of active colloids to certain orientation angles. This effect is dependent on the imposed flow rate, with higher flows leading to more robust angular confinement.4 Our findings could have implications for developing applications involving the guiding of self-propelled particles in microfluidic channels.

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