Fig. 3.

Intensity–height curves obtained for the a 3-um diameter bead in water for various NaCl concentrations (corresponding Debye lengths: 1 nm, 5 nm, and 20 nm). The blue curve shows a condition of insufficient screening: Despite the optical tweezer focus moving beneath the surface of the glass, the negatively charged bead is repelled electrostatically at close separations and unable to actually approach the negatively charged surface. Green and red curves: Additional salt brings calibration curves closer to the ideal exponential behavior.