Active matter

Active colloids in optical environments

Over the last decade, active colloids have become a critical model for examining collective behaviour in biological systems. Unlike their natural counterparts, these synthetic systems benefit from rapid dynamics, are adjustable, and their small size facilitates easier data collection. They have been key in studying individual interactions’ effects on group formation, driven by forces like attraction, repulsion, alignment, reorientation, vision, and communication. The impact of environmental physical attributes on group formation, including obstacles and disordered potentials, has also been explored. While heterogeneous resource distribution critically influences biological system dynamics, its effect on collective phenomena is not yet fully understood.

In my work, I focus on understanding the impact of complex energy distributions on group dynamics. Light represents a unique tool to create controlled environments at the microscale. Specifically, two-dimensional energy landscape with adjustable spatial complexity can be obtained by projecting laser light through an optical diffuser, creating a diffraction pattern composed of randomly positioned light patches (known as speckle). When moving in two-dimensional complex environments, the active colloids tend to avoid the highest energy patches by aligning perpendicular to the gradient of the energy field. This property has an important effect also on the collective behaviour of the colloids. In particular, active colloids are known to form groups when, upon collisions, slow down and cluster until the propulsion direction of one of the individuals points outside the cluster. The complex landscape restricts the size of groups, as clusters tend to grow until they fill the space confined between adjacent high-energy patches. Moreover, in a complex environment, groups have a higher stability, as shown by the reduced exchange of individuals compared to a homogeneous landscape. This increased stability stems from: i) reduced encounters due to prolonged stays in low-energy areas, decreasing aggregation rates; ii) within clusters, colloids orient towards lower energy, causing them to face inward the cluster and reducing the likelihood of fragmentation. Importantly, devoid of biological complications, this model system could provide insights into the importance of patchy landscapes in living active matter dynamics and it also holds implications for refining colloidal self-assembly.

if you want to know more:

• a laymen discussion on my results can be found in my blogpost;
• more details and references can be found in my arXiv preprint;
• a general intro on active matter is provided by this review.

main collaborators:

• Giorgio Volpe (UCL, London);
• Hartmut Löwen (HHU, Düsseldorf)

Phototaxis of microalgae in complex photonic environments 

Microalgae (Chlamydomonas reinhardtii) can orient themselves in light fields, a property called phototaxis. As opposed to chemotaxis (the capacity to navigate chemical fields) which has been extensively studied in the past 50 years, this phenomenon, mediated by a specialised organelle called the eyespot, is still poorly understood. By studying the swimming behaviour of microalgae in tailored laser illuminations, we aim to offer new insights into how Chlamydomonas modulate their motility in response to light and potentially propose novel strategies for controlling and redirecting their movement.

While you wait for more on this, check the website of Raphaël Jeanneret, where you will find out more about microalgae.

Ecology: relationship between home range and population density in mammals

My interest in applying physics concepts to different scales got me involved in the study of ecological systems. In collaboration with Prof. Santini (Sapienza University, Italy), where we analysed large ecological datasets to study the relationship between the home range (area in which an animal lives) and population density in mammals. In short, I generalised the so-called gas model—a simple mechanistic model that is widespread in the ecology community and where the frequency of interaction among animals is approximated by the collisions among gas particles—to depict different ecological and environmental traits. More on this will hopefully be out soon!