Crowds of tiny particles disperse as their environment becomes more disordered, according to a study by scientists from University College London (UCL), Bilkent University and Université Pierre et Marie Curie. The new mechanism is counterintuitive and might help describe crowd behavior in natural, real-world systems where many factors impact individuals’ responses to either gather or disperse.
“Bacterial colonies, schools of fish, flocking birds, swarming insects and pedestrian flow all show collective and dynamic behaviors that are sensitive to changes in the surrounding environment, and their dispersal or gathering can be sometimes the difference between life and death,” said lead researcher Dr. Giorgio Volpe of UCL.
“The crowd often has different behaviors than do the individuals within it, and we don’t know what the simple rules of motion are for this. If we understood these and how they are adapted in complex environments, we could externally regulate active systems. Examples include controlling the delivery of biotherapeutics in nanoparticle carriers to the target in the body, or improving crowd security in a panic situation.”
The study, published earlier this month in Nature Communications, investigated the behavior of active colloidal particles in a controllable system to find out the rules of motion for individuals gathering or dispersing in response to external factors.
Colloidal particles are free to diffuse through a solution, and for this study suspended silica microspheres were used. The colloidal particles became active with the addition of E. coli bacteria to the solution. Active colloidal particles were chosen as a model system because they move of their own accord using the energy from their environment, which is similar to how animals move to get food.
Initially, the active colloidal particles gathered at the center of the area illuminated by a smooth beam, which provided an active potential. Disorder was introduced using a speckle beam pattern, which disordered the attractive potential and caused the colloids to disperse from the area at a rate of 0.6 particles per minute over 30 minutes. The particles switched between gathering and dispersing proportionally to the level of external disorder imposed.
Erçağ Pinçe, who with Dr. Sabareesh K. P. Velu (both of Bilkent University) is first author of the study, said: “We didn’t expect to see this mechanism, as it’s counterintuitive, but it might already be at play in natural systems. Our finding suggests there may be a way to control active matter through external factors. We could use it to control an existing system, or to design active agents that exploit the features of the environment to perform a given task, for example, distinct depolluting agents for different types of polluted terrains and soils.”
Coauthor Dr. Giovanni Volpe, also of Bilkent University, added: “Classical statistical physics allows us to understand what happens when a system is at equilibrium, but unfortunately for researchers, life happens far from equilibrium. Behaviors are often unpredictable, as they strongly depend on the characteristics of the environment. We hope that understanding these behaviors will help reveal the physics behind living organisms, and also help deliver innovative technologies in personalized health care, environmental sustainability and security.”
The team now plans on applying their findings to real-life situations to improve society. In particular, they want to exploit the main conclusions from their work to develop intelligent nanorobots for applications in drug delivery and environmental sustainability that are capable of efficiently navigating through complex natural environments.
Funding for the study was provided by the European Research Council, Marie Curie Career Integration Grants and the COST Action – Flowing Matter, and the Scientific and Technological Research Council of Turkey.