Guiding Artificial Self-Propelling Particles

Active Materials consist of a collection of objects that self-propel by absorbing energy from either an internal reservoir or from their surroundings. Examples of these include swimming bacteria, while artificial microswimmers can be realised by active Janus colloids. In recent years, they have become a topical research direction in soft matter due to the possibility to realise self-controlling applications. In these materials, the building blocks can carry out tasks autonomously, without an outside intervention. Examples of these include active colloidal materials for both lab-on-chip and drug delivery applications in microscale and autonomous robotic applications in macroscale. A grand challenge in active matter research, is how to guide and control the self-motile building blocks. Understanding how to do this gives novel possibilities to design artificial materials with life like qualities for robotic (dry; macroscale) and novel colloidal (hydrodynamic; microscale) applications. One possibility is to use confining surfaces. Both, spherical self-propelling colloids and active granular rods have been shown to accumulate at surfaces. And various geometrical constructions can be used to guide them. Further, the confinement can become motile, due to aggregation of the self-motile objetcs. This can give a rise to novel applications in swarm-robotic applications, where a the collections of a simple non-communicating build blocks are used to create coherent collective motion. However, these approaches are inherently 2-dimensional. To realised guiding in the 3-dimensional bulk, the properties of the swimming media can be used. Recently, it has been shown that microswimmers can be guided using the broken orientational symmetry of nematic liquid crystals. Typically, rod like bacteria will align along the nematic director and they can be guided into complex paths by controlling the direction of the nematic order. This project deals with guiding artificial self-propelling particles It includes two distinctive branches: 3-dimensional guiding spherical microswimmers using fluid topology (liquid crystalline order) for microscale applications and granular active particles (self-propelling rods) in flexible containers for 2-dimensional robotic applications. In the first part, we will construct a complete hydrodynamic picture of spherical (colloidal) microswimmers in liquid crystals. In addition to classic nematic phase, we will consider both also chiral nematic and blue phases. We will interface state-of-the art simulations (lattice Boltzmann) with analytical calculations. We will study both the hydrodynamic alignment of the particles as well as the trapping and swimming along defect lines. In the second part, we will consider dry granular rods in a flexble container, which is rendered motile through the accumulation of the active rods in high curvature regions.