  Organisms often swim through density stratified fluids. In this Letter, we
investigate the dynamics of small active particles swimming in density
gradients and report theoretical evidence of taxis as a result of density
stratification ($\textit{densitaxis}$). Specifically, we calculate the effect
of density stratification on the dynamics of a force-free spherical squirmer
and show that density stratification induces reorientation that tends to align
swimming either parallel or normal to the gradient depending on the swimming
gait. In particular, particles that propel by generating thrust in the front
(pullers) rotate to swim parallel to gradients and hence display (positive or
negative) densitaxis, while particles that propel by generating thrust in the
back (pushers) rotate to swim normal to the gradients. This work could be
useful to understand the motion of marine organisms in ocean, or be leveraged
to sort or organize a suspension of active particles by modulating density
gradients.
