Taxis in density gradients

Microorganisms living in oceans, lakes, or ponds (e.g., planktonic organisms) often move through density gradients caused by spatially varying temperature or salinity. These 'active particles' usually perform vertical migration in search of food and this motivated the earlier researchers to address the influence of vertical gradients on the vertical motion of particles. Here we discuss our analysis on the effect of density gradients on particle reorientation which, among other things, determines whether vertical migration is stable. Our analysis reveals that active squirmer-type particles reorient to swim either vertically or horizontally in vertical density gradients. Pushers like E. coli reorient to swim horizontally, whereas pullers, like Chlamydomonas reinhardtii, reorient to swim vertically but up vs down gradients depends on their initial orientation. These results point towards applications of active matter control through prescribed density gradients.