  Mechanical characteristics of single biological cells are used to identify
and possibly leverage interesting differences among cells or cell populations.
Fluidity---hysteresivity normalized to the extremes of an elastic solid or a
viscous liquid---can be extracted from, and compared among, multiple
rheological measurements of cells: creep compliance vs. time, complex modulus
vs. frequency, and phase lag vs. frequency. With multiple strategies available
for acquisition of this nondimensional property, fluidity may serve as a useful
and robust parameter for distinguishing cell populations, and for understanding
the physical origins of deformability in soft matter. Here, for three disparate
eukaryotic cell types deformed in the suspended state via optical stretching,
we examine the dependence of fluidity on chemical and environmental influences
around a time scale of 1 s. We find that fluidity estimates are consistent in
the time and the frequency domains under a structural damping (power-law or
fractional derivative)model, but not under an equivalent-complexity
lumpedcomponent (spring-dashpot) model; the latter predicts spurious time
constants. Although fluidity is suppressed by chemical crosslinking, we find
that adenosine triphosphate (ATP) depletion in the cell does not measurably
alter the parameter, and thus conclude that active ATP-driven events are not a
crucial enabler of fluidity during linear viscoelastic deformation of a
suspended cell. Finally, by using the capacity of optical stretching to produce
near-instantaneous increases in cell temperature, we establish that fluidity
increases with temperature---now measured in a fully suspended, sortable cell
without the complicating factor of cell-substratum adhesion.
