ICTS

The term colloid stems from the Greek word ‘Kolla’ for glue, combined with the ending ‘oid’ for forming and describes a mixture consisting of one non-crystalline substance dispersed in a second substance. Colloids include for example gels, sols, smoke and emulsions and a number of techniques have been established to produce and characterize this fascinating state of matter. In this first class we will look at different techniques to produce particles and droplets and learn how to manipulate them.
 

... or to enable them to convert energy from one form to another in order to move or exert force, we need to push them out of equilibrium. While this broad definition encompasses all living systems, artificial 'out-of-equilibrium' systems have become the focus of interdisciplinary research over the last two decades. In this talk we will explore different ways to create 'artificial active matter' and the different properties that result from the different motion mechanisms.

While the behaviour of biological microswimmers is undoubtedly influenced by physics, it is often controlled and manipulated by active sensing processes. Understanding the respective influences of the environment can help to engineer the desired response in artificial swimmers. In most cases, the achievement of biomimetic behaviour requires an understanding of the swimming mechanisms of both biological and artificial microswimmers and the parameters that induce mechanosensory responses. Using different examples of tactic behaviour, I will show how active matter can be tuned to mimic bacteria or other microorganisms.

Genotypically similar cells of tissues show phenotypic heterogeneity dictated by differential protein expression patterns. In epithelia, this biochemical heterogeneity is further complexed by a physical heterogeneity, dictated by cellular contractility and matrix stiffness. However, for epithelial tissues, such inherent bio-physical heterogeneity in space and time remains uncharacterized, and its relevance to tissue function remains unknown. In this presentation, I will discuss our work on how biochemical and physical stochasticity in epithelial cells dictate tissue function. I will focus on tissue remodelling during wound healing, and on mutant cell extrusion during epithelial defence against cancer, and demonstrate how both these functions require bio-physical heterogeneity. I will also discuss how spatial heterogeneity is regulated in time, and how deregulated biological oscillators might influence tissue heterogeneity and subsequently its function.

Microtubule filaments are an important example of active polymer where the motor protein (kinesin) make use of chemical energy derived from the hydrolysis of adenosine triphosphate (ATP) to induce the conformational changes, which then leads to its motion on microtuble. In our work, we are interested in studying such active polymer that exhibits various interesting dynamics like self-propulsion, swelling, shrinkage, loop formation, spontaneous oscillation, spiral formations, enhanced diffusion etc.

Active turbulence — the spatio-temporally complex motion of a dense suspension of microorganisms such as bacteria — has gathered great traction recently as an intriguing class of emergent, complex flows, occurring in several living systems at the mesoscale, whose understanding lies at the interface of non-equilibrium physics and biology. However, are these low Reynolds number living flows really turbulent or just chaotic with structural, or even superficial, similarities with high Reynolds number (classical) inanimate turbulence? This is a vital question as the fingerprints of classical turbulence —-- universality, intermittency and chaos —-- makes it unique amongst the many different driven-dissipative systems. In this talk we address these questions with a focus on the issues of (approximate) scale-invariance, intermittency and maximally chaotic states and how they lead to anomalous diffusion in bacterial suspensions. In particular, we show the existing of a critical level of activity beyond which the physics of bacterial flows become universal, accompanied by maximally chaotic states which allow for efficient, Levy-walk mediated foraging strategies.

Unraveling the motion of microorganisms in dilute and porous media is important for our understanding of both the molecular basis of their swim gait and their survival strategies in microbial habitats. First, I will show that by using renewal processes to analyze experimental measurements of wild-type {\it E. Coli}, we can provide a quantitative spatiotemporal characterization of their run-and-tumble dynamics in bulk. We further demonstrate quantitatively how the persistence length of an engineered strain can be controlled by a chemical inducer and characterize a transition from perpetual tumbling to smooth swimming. Second, I will address how this run-and-tumble gait evolves towards a hop-and-trap motility pattern of agents moving in a porous environment. Using computer simulations, we discover a geometric criterion for their optimal spreading, which emerges when their persistence lengths are comparable to the longest straight path available in the porous medium. Our criterion provides a fundamental principle for optimal transport in densely-packed environments, which could be tested experimentally by using engineered cells and may provide insights into microbial adaption mechanisms.  

A more specialized talk focused on the effects of (spatial) quenched disorder on the collective dynamics of the VM