Active systems, composed of “particles” that consume local energy to perform work, have attracted a great deal of attention due to their relevance in Physics, Biology, Medicine, and Engineering. Examples of these systems can be found at vastly different length scales: from the nano-scale, with kinesin motors transporting cargo inside of cells, to the micro-scales of cells crawling around to close wounds or bacteria swimming in viscous media, and finally, to the macro-scales at which fish, birds, and humans move about. In our work, we have focused on studying the dynamics of micro-meter sized active particles, including both swimmers (e.g., bacteria) and crawlers (e.g., epidermal cells). While we have a fairly complete understanding of the propulsion mechanism used by such particles, the non-trivial coupling between the particle and its environment gives rise to complex dynamical behaviors that have yet to be fully explained. In other words, we know how a single bacteria or cell is able to move, but we cannot always predict what will happen when many of these particles come together. Given the difficulty of performing controlled experiments on these type of systems, computer simulations have become one of the preferred approaches for studying the properties of these active systems. We will introduce the basic computational models that allow us to study the dynamics of interacting swimmers, including the full hydrodynamic interactions, as well as the collective motion of crawling cells on 2D substrates. In the first part of the presentation, we will discuss the collective motion of particles swimming in a viscous medium. We will show that the type of swimming, determined by whether the propulsion is generated at the front (e.g., a puller like the Chlamydomonas algae) or at the back (e..g,, a pusher such as spermatozoids or most bacteria), has a crucial effect on the hydrodynamic interactions between swimmers, and thus, on the collective motion that can be observed[1-4]. In the second part of our talk, we will consider the dynamics of cells crawling on 2D substrates. Here, we will focus on the response of the cell to a periodic stretching of the substrate, which is known to result in a preferential alignment that is cell specific[5], and on the role of cell-cell interactions on the large scale collective motion of cell colonies[6].

References:
[1] Molina, Nakayama, and Yamamoto, Soft Matter 9, 4923 (2013)
[2] Molina and Yamamoto, Mol. Phys. 112, 1389 (2014)
[3] Oyama, Molina, and Yamamoto, Phys. Rev. E 93, 043114 (2016)
[4] Delfau, Molina, and Sano, Europhys. Lett. 114, 24001 (2016)
[5] Okimura, Ueda, Sakumura, and Iwadate, Cell Adhes. Migr. 0, 1 (2016)
[6] Schnyder, Molina, Tanaka, and Yamamoto, Sci. Rep. 7, 5163 (2017)