Dependence of active rheological effects on swimmer type and matrix state

Determine whether the sign of the active contribution to rheology is jointly controlled by the swimming mechanism of the microswimmers and the mechanical state of the passive matrix, as suggested by the contrasting fluidization produced by pusher-type swimmers in simple suspensions and rigidification produced by puller-type Chlamydomonas reinhardtii in jammed emulsions.

Background

The paper demonstrates that motile puller-type Chlamydomonas reinhardtii reinforce a jammed emulsion by increasing its yield stress, whereas prior work has shown that pusher-type swimmers can reduce viscosity and fluidize simple suspensions. This contrast motivates an unresolved broader question about whether the direction of activity-induced rheological change depends on both swimmer propulsion mechanism and the state of the passive material.

Answering this question would require systematic comparisons involving other microswimmer classes, including pusher-type bacteria, across passive matrices with different structural and mechanical states. The paper identifies this as a direction for future investigation but does not resolve it.