(407h) Dynamic Self-Assembly and Swarming in Ensembles of Camphor Boats

Soh, S., Northwestern University
Branicki, M., Northwestern University
Grzybowski, B., Northwestern University

Dynamic self-assembly (DySA), that is, self-assembly in systems organizing only when dissipating energy, is of fundamental interest in the context of life, and can have practical applications in new classes of smart/adaptive systems and materials. Swarming of these DySA systems provides addition capability to explore space while performing intelligent functions. However, while swarms are ubiquitous in biological systems (bacteria, fish, ants, etc.), examples of artificial collective movers are largely limited to complex robotics systems. Here, we describe a simple, camphor-in-gel system where the gel particles not only perform DySA into regular structures, but also swarm collectively.

Experiments were done with multiple millimeter-sized camphor-in-gel particles floating on an air-water interface in a Petri dish with varying depths of water. Camphor is a well-known surface active chemical used widely to propel objects or ?boats? through an air-water interface. Although the motion of individual camphor ?boat? has been studied extensively for several decades, multiple ?boats? and the interactions between them remain undescribed. Experiments show that the interactions between the multiple particles are nontrivial: at high level of water, the interaction is purely repulsive, while at low level of water, the interaction can be both repulsive and attractive. This novel type of interaction (similar to the Lennard-Jones potential between molecules) at low water level originates from the convection rolls driven by the surface-active agent, camphor. Numerical solution of the coupled Navier-Stokes equation with the transport equation of camphor reproduced these experimental observations. These inter-particle interactions enable the emergence of many surprising collective behaviors, such as DySA, autonomous swarming and ?chemotactic? motion toward gradients of camphor concentration, temperature and water depth. The swarms can also be controlled to move forward or backward depending on the depth of water in the dish. This simple, artificial system demonstrates the capability of performing controlled, intelligent functions through employing appropriate interactions between the individual components. The emergence of these non-intuitive behaviors also appeals for more exploration and fundamental understanding of surface tension driven flow in thin layers of water in general.