Fringe pattern photobleaching

II.1. Experimental Procedure. The micelles self-diffusion coefficient has been determined by fluorescence recovery after fringe pattern photobleaching (FRAP). A fluorescent probe was incorporated in the micelles it was checked that its finite residence time in the micelle did not affect the diffusion process. 

Diffusion coefficients of the tracer molecules have been measured also by fluorescence recovery after fringe pattern photobleaching recovery. The fluorescence recovery curves are pure exponentials but the diffusion... 

Unambiguous determination of the conditions under which slippage occurs requires a technique able to measure the velocity of the fluid in the immediate vicinity of the solid wall over a thickness comparable to the size of a polymer chain, i.e. a few tens of nanometers. Classical laser Doppler velocimetry does not meet this requirement even if it allows for the determination of velocity profiles which clearly reveal a non-zero velocity within typically a few 10 pm from the wall. We have developed a new optical technique. Near Field Velocimetry (N.F.V.) [14], which combines Evanescent Wave Induced Fluorescence (E.WF.) [27] and Fringe Pattern Fluorescence Recovery After Photobleaching (F.P.F.R.A.P.) [28]. The former technique gives the spatial resolution normal to the solid wall, while the latter one enables the determination of the local velocity of the fluid. A major constraint of the technique is that it needs polymer molecules labelled with an easily photobleachable fluorescent probe. 

Due to chain entanglements the self diffusion coefficient of the labeled macromolecules is very small (<10 mVs), and the transport of the photobleached tracers takes place predominantly through convection. If there is no slip at the polymer - prism interface, the fluorescence intensity decreases steadily with time as the concentration pattern is progressively tilted by the shear. If slip occurs, the concentration pattern is translated (and tilted) in front of the interference fringes creating a damped periodic oscillation of the fluorescence intensity with a period T = iA s, if Vs is the average slip velocity within the distance A from the prism surface. Measuring T and i yields directly Vj. 

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