MixQUIckR: Mixing with QUIncke Rollers

The problem of mixing Mixing is an easy concept to grasp in day to day life. It might involve simple liquids, like syrup in water, or more complex materials and situations, like the preparation of concrete or the optimisation of drug production rates in the pharmaceutical industry. However, understanding how mixing works and how to optimise it is a difficult task. At small length scales (on the order of the millimeter or smaller), mixing in liquids is further complicated by the fact that the flow is reversible. This means that if one tries to mix by stirring in one direction then reversing the direction, the liquids will unmix. One way of solving this problem is to use small particles that will be able to stir the liquid without retracing their steps, therefore without unmixing the liquid. The Quincke effect and Quincke rollers When a non conductive object is immersed in a conductive liquid and subjected to a high enough electric field, it will start rotating. This is known as the Quincke effect. If these objects are dense spheres, they will settle on the bottom of the liquids container, and when subjected to the Quincke effect, their rotation will lead them to roll on the surface they settled on. Such objects are called Quincke rollers. They can reach high velocities: particles with a diameter of 5 microns can cover a distance of 200 times their size in one second. When the system contains a lot of particles, they can undergo collective motion: they can for example form flocks or vortexes. Using Quincke rollers to mix at small scales The aim of this project is to experimentally study the ability of Quincke rollers to mix their surrounding liquid. We will first examine what happens on a height of one particle diameter, where the particles movement will stir the fluid. Our preliminary calculations suggest that mixing in these conditions should happen quite fast. Second, we will try to understand the effect of collective behavior on mixing, while staying on a height of one particle. Finally, we will explore what happens in the liquid above the particles.

The problem of mixing Mixing is an easy concept to grasp in day to day life. It might involve simple liquids, like syrup in water, or more complex materials and situations, like the preparation of concrete or the optimisation of drug production rates in the pharmaceutical industry. However, understanding how mixing works and how to optimise it is a difficult task. At small length scales (on the order of the millimeter or smaller), mixing in liquids is further complicated by the fact that the flow is reversible. This means that if one tries to mix by stirring in one direction then reversing the direction, the liquids will unmix. One way of solving this problem is to use small particles that will be able to stir the liquid without retracing their steps, therefore without unmixing the liquid. The Quincke effect and Quincke rollers When a non conductive object is immersed in a conductive liquid and subjected to a high enough electric field, it will start rotating. This is known as the Quincke effect. If these objects are dense spheres, they will settle on the bottom of the liquid's container, and when subjected to the Quincke effect, their rotation will lead them to roll on the surface they settled on. Such objects are called Quincke rollers. They can reach high velocities: particles with a diameter of 5 microns can cover a distance of 200 times their size in one second. When the system contains a lot of particles, they can undergo collective motion: they can for example form flocks or vortexes. Summary of the project The aim of this project was to experimentally study the ability of Quincke rollers to mix their surrounding liquid. Our first idea was to use dyed liquid to visualize the liquid's movement due to the action of the particles. However, this induced an electrohydrodynamic instability in the system. We therefore turned to trying to solve this technical limitation in order to continue the project. Since the Principal Investigator was hired as a Maitresse de Conférences (equivalent to Assistant Professor in France) only 9 months after the beginning of this funding, the project is still at a stage where a solution to the visualization problem needs to be found.