(a) Simplified schematic layout of the optical tweezers system. (b) Sketch showing the trapped bead (radius r) with respect to the surface at height h. (c) Ray-tracing representation of the refracted trapping beam, showing a shift between the marginal rays and the paraxial rays, which is responsible for an elongated focal volume (spherical aberration).
Variation of the apparent drag force as a function of the distance to the wall, as deduced from measurements performed after calibrating the trap force at a distance of 20 μm from the wall, with (solid lines, closed symbols) or without (dashed lines, open symbols) correction for the hydrodynamic wall effect, at different laser powers in the sample.
Drag force correction for yeast cells as function of the distance to the wall, obtained by using Eq. (9) (solid line) and by CFD (sphere, open circles; prolate spheroid, solid triangles). Inset: relative difference between CFD results and results obtained using Eq. (9).
(a) Wall-corrected drag force (pN) vs. laser power at the objective, measured at a distance of 3 μm (red closed circles) and 20 μm (black closed squares) from the surface. Experimental data taken at 20 μm and converted to the distance of 3 μm (open circles) though multiplication by β. (b) Conversion factor β described by Eq. (13).
Micrographs showing budding of a cell in the trap: time zero, Tin situ = 30.1 °C (a); after 3 h, Tin situ = 30.3 °C, before (b) and after release and trapping again (c); after 5 h, Tin situ = 30.2 °C, before (d) and after release and trapping again (e). Scale bar, 5 μm.
Zeta potential (mV) as a function of ionic strength (mM) for (a) the glass particles and (b) the yeast cells, in the presence of NaCl (open symbols) or CaCl2 (closed symbols). Dashed lines are decaying exponential fits.
Proportion of firmly, mildly, and non-adhering cells (%) ratioed to the total population, measured at 1 h contact time, at different CaCl2 IS (50, 80, 100, 150, 200, and 330 mM) and NaCl IS (150, 180, 200, 250, and 330 mM).
Proportion of mildly adhering cells (in a.u., normalized by the maximum value) removed after 1 h contact time for different intervals of the applied force (pN) at different CaCl2 IS (50, 80, 100, 150, 200, and 330 mM) and NaCl IS (150, 180, 200, and 250 mM). Inset: plots of the median detachment forces F* (pN) vs. (mM0.5) with Ca and Na with linear fits (solid lines).
Potential energy (left axis, 103 kT, solid line) and force (right axis, 103 pN, dashed line) computed by the DLVO theory as function of separation distance (nm) in NaCl 150 mM solution. Inset is a close-up of the figure.
Values (mean ± standard deviation (SD)) of yeast cell dimensions a and b of related parameters and of shape-drag correction factor Ks .
Force level toward the barrier (nN), energy barrier (kT) and force level towards the secondary minimum (pN) as function of IS. nb, no barrier.
Comparison of single-cell techniques (modified from Ref. 68).
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