Fig. 1.

Schematic of experimental approach. Laser trap stiffness, ${\kappa }_{trap},$ is first measured by trapping a particle of known diameter in water (A), and then determining the corner frequency from a Lorentzian fit to the power spectral density of particle fluctuations (B). ${\kappa }_{trap}$ is related to the corner frequency by Eq. (2). The procedure is repeated in the center of an excised vessel slice (C), where the corner frequency of the power spectral density (D) is used to determine the viscosity,${\eta }_0$of the culture media by Eq. (2). The apparent viscosity, ${\eta }_i$as a function of gap thickness is determined by optically measuring the rotation rate of a vaterite particle rotating under constant optical torque at various distances from the vessel wall (E). For each gap thickness, ${\eta }_i$ is calculated by measuring vaterite particle rotation rate and Eq. (5) (F).