Trapping of Swimming Microorganisms at Lower Surfaces by Increasing Buoyancy

Models suggest that mechanical interactions alone can trap swimming microorganisms at surfaces. Testing them requires a method for varying the mechanical interactions. We tuned contact forces between Paramecia and surfaces in situ by varying their buoyancy with nonuniform magnetic fields. Remarkably, increasing their buoyancy can lead to $\sim 100\%$ trapping at lower surfaces. A model of Paramecia in surface contact passively responding to external torques quantitatively accounts for the data implying that interactions with a planar surface do not engage their mechanosensing network and illuminating how their trapping differs from other smaller microorganisms.

Figure 1

(a) The surface trapping probability (STP) of P. tetraurelia at the bottom. Frame showing 3 P. tetraurelia at the bottom surface when $w/w_{1g}=-2$. The long axes of their bodies were canted away from horizontal as they swam along the surface. (b) STP of P. tetraurelia near the bottom surface as $w/w_{1g}$ changes from 1 to $-2$. The negative force indicates that the force direction is from the bottom to the top. (c) STP of P. tetraurelia and $w/w_{1g}$ (solid line) as a function of time. $w$ was changed in steps. (d) Trapping of P. caudatum at the bottom surface. Upward apparent weight force ($w/w_{1g}=-6$) is applied. (e) STP of P. caudatum at the top (triangle) and bottom (downward triangle) surface. (f) STP of P. caudatum at the top surface minus the bottom surface and $w/w_{1g}$ as a function of time. The solid line marks the changes in $w/w_{1g}$. The dotted line denotes where both the probability and $w/w_{1g}$ are zero. The bars on the points give the uncertainty estimated presuming the measurements followed a binomial distribution.

Figure 2

STP at the top (triangle) and bottom (downward triangle) surface as a function of homogeneous magnetic field at constant apparent weight of $w/w_{1g}=1$. The bars on the points give the uncertainty estimated presuming the measurements followed a binomial distribution.

Figure 3

(a) A sketch of a canted Paramecium in contact with and swimming along the bottom surface. In this figure, the Paramecium swims against $\overrightarrow{w}$. (b) Phase diagram of P. caudatum behavior at the bottom surface. The upper four frames give schematics of the swimming behavior in the magnetic field regions I–IV. The gray region specifies the range of ${\theta }_{\max }$ expected for a typical population of swimmers.