Feeders and Expellers, Two Types of Animalcules With Outboard Cilia, Have Distinct Surface Interactions*

Within biological fluid dynamics, it is conventional to distinguish between 

"puller" and "pusher" microswimmers on the basis of the forward or aft 

location of the flagella relative to the cell body: typically, bacteria are

pushers and algae are pullers.  Here we note that since many pullers have 

"outboard" cilia or flagella displaced laterally from the cell centerline on both

sides of the organism, there are two important subclasses whose

far-field is that of a stresslet, but whose near field is qualitatively more complex.  

The ciliary beat creates not only a propulsive force

but also swirling flows that can be represented by paired rotlets with two possible

senses of rotation, either "feeders" that sweep fluid toward the cell apex, or 

"expellers" that push fluid away.  Experimental studies of the 

rotifer Brachionus plicatilis in combination with earlier work on the green algae 

Chlamydomonas reinhardtii show that the two classes have markedly

different interactions with surfaces.  When swimming near a surface, expellers such as 

C. reinhardtii scatter from the wall, 

whereas a feeder like B. plicatilis stably attaches.  This results in a stochastic "run-and-stick" locomotion, with periods of 

ballistic motion parallel to the surface interrupted by trapping at the surface.