Optical Tweezers Laboratory

Monitoring of fluid properties with confocal tweezer microscope

A particle trapped in a moving tweezer beam will follow with a time lag because of viscous drag. If the tweezer moves sinusoidally with amplitude a the particle will move sinusoidally with a phase lag.  The phase lag is determined by the amplitude a as well as the physical properties of the trap and the viscosity of the fluid in which the particle moves.  Neglecting inertial forces, the motion of the particle is determined by a and the parameter C = a/wb, where a is the trap stiffness (proportional to laser power), w is the oscillation frequency of the trap, and b is the viscous drag coefficient.  Below is a link to an animation of a bead interacting with a trap while the parameter C decreases, to model an experiment in a material whose viscosity increases over time, such as a gelling polymer.

In our experiment, we do lock-in confocal detection at the second harmonic of the signal reflected from a trapped particle. The measured phase can be used to determine C, and hence fluid properties at the position of the trap.

The animation displays the current value of C, as well as the phase (ph) of the second harmonic signal in degrees.

The next animation (974kb) includes the effects of Brownian motion.  The value of C is 0.07 for oscillation amplitude 1.9 beam waists, 1kHz oscillation frequency, 1mm diameter bead in water. Animation including Brownian motion

Recently, we have been tethering the trapped object to the coverslip using lambda DNA to eliminate the need for axial trapping and allow the use of low numerical aperture trapping lenses.  The tether stabilizes the trapped particle against ejection from the trap by longitudinal radiation pressure. In this way, long working distances become possible and the expense of the optics is reduced.

Here is a Powerpoint Presentation giving further details (SPIE Annual Meeting, August 2004 paper 5514-64).