Active Fluids: How a Collection of Worms Create a Liquid

Fluids consist of a collection of particles that interact through collisions. Fluid properties, such as the flow resistance or viscosity, emerge from the many interactions such passive particles (read: molecules) have. For example: a classic result is that the viscosity of an ideal gas grows with the square root of its temperature. One can wonder what kind of fluid properties emerge from the multitude of interactions between active particles. Active particles can be large (cars in a traffic jam) or small (bacteria). What they have in common among each other is that they have an internal "motor"; they consume energy and can propel themselves. This feature also sets active particles apart from passive particles such as molecules, which require external forcing from for example an energy bath (thermal fluctuations) to move and collide with other particles. Obviously cars on a highway or worms in a bucket have very different flow behavior from ordinary liquids such as water. Can we quantify these differences?

In this project we investigate this question further by studying the collective behavior of a collection of worms. We can breed these worms ourselves or simply buy them. The worms can be studied on an individual basis, to get a sense of how they move by themselves. We can also do flow studies on them, for example by measuring how fast they move through a funnel, or how they flow in a flow measuring device called rheometer. There are many open questions to be answered, so there is a lot of space for creative ideas. One application is to develop new sensing tools to measure the activity of insect colonies, which is of direct relevance to the growing industry of insect breeding for human/animal nutrition.

This project is available for both Bachelor and Master students; Soft Matter or Advanced Soft Matter are very helpful courses to provide background knowledge. However, this project is very flexible and is suitable for all students with a creative and open mind.

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Figure 1: Animated gif showing the tracks of individual worms. (Double click the image to view animation) The microscopic dynamics extracted from these data can be linked to the flow behavior of a collection of worms.