Fig. 4: Microrheology with an optical trap and multiplane detection.

3D particle tracking mode in an aqueous solution. a An optically trapped microsphere is imaged in 9 planes simultaneously (planes represented by dashed lines). b Captured image of nine separate Z planes (Δz = 0.79 μm). The planes (labelled from number −4 to +4) are simultaneously recorded at the camera sensor to extract the 3D trajectory. c The resulting 3D trajectory of optically trapped microsphere in water with individual positions depicted as blue circles (down sampled for clarity from a total of 2 × 10⁵ frames). d The normalised position autocorrelation function (NPAF) versus dimensionless time \(\tau^\star\) provides the relative viscosity (dotted line) ‘at a glance’. e The relative viscosity measured at each position over a range of depths.