(465f) Densitaxis : Active Particle Motion in Density Gradients

Living (and synthetic) microorganisms frequently move through inhomogeneous environments like gradients in heat, light, nutrients, chemical concentration, temperature or salinity. They respond to these inhomogeneities by adjusting their speed and orientation, swimming dynamics can be significantly complicated due to these interactions but they can also lead to directed motion termed taxis. For example, E. coli bacteria reorient to swim up the nutrient gradients but down light gradients. Here we introduce the concept of “densitaxis”, which describes the orientational dynamics and directed motion resulting from interactions with fluid density gradients (caused by temperature or salinity gradients). These dynamics are sensitive to whether the organism generates thrust in front (so-called pullers) or in the back (pushers) as each distinctly alters the background density field. Ultimately, we find that pullers, such as Chlamydomonas reinhardtii, reorient to swim up or down the density gradients depending on their initial orientation, while pushers like E. coli rotate to swim normal to the gradients. This newly discovered taxis could provide insight into the motion of marine organisms in oceans where density gradients are prevalent or be exploited to sort and organize a dilute suspension of organisms but may also lead to complex nonlinear dynamics when hydrodynamic interactions are considered.