Fluorescence photobleaching to evaluate flow velocity and hydrodynamic dispersion in nanoslits

Abstract

Velocity measurement is a key issue when studying flows below the micron scale, due to the lack of sensitivity of conventional detection techniques. We present an approach based on fluorescence photobleaching to evaluate flow velocity at the nanoscale by direct visualization. Solutions containing a fluorescent dye are injected into nanoslits. A photobleached line, created through laser beam illumination, moves through the channel due to the fluid flow. The velocity and effective diffusion coefficient are calculated from the temporal data of the line position and width respectively. The measurable velocity range is only limited by the diffusion rate of the fluorescent dye for low velocities and by the apparition of Taylor dispersion for high velocities. By controlling the pressure drop and measuring the velocity, we determine the fluid viscosity. The photobleached line spreads in time due to molecular diffusion and Taylor hydrodynamic dispersion. By taking into account the finite spatial and temporal extensions of the bleaching under flow, we determine the effective diffusion coefficient, which we find to be in good agreement with the expression of the two dimensional Taylor-Aris dispersion coefficient. Finally we analyze and discuss the role of the finite width of the rectangular slit on hydrodynamic dispersion.

Impact of saturation on dispersion and mixing in porous media: Photobleaching pulse injection experiments and shear‐enhanced mixing model

“…[]. The first one, and the most remarkable, is the use of a laser diode ( LBX Oxxius ) of wave length 494 nm and power 150 mW to create the initial pulse line injection by desactivating the Fluorescein molecules, a technique called photobleaching [ Cuenca and Bodiguel , ]. The photobleaching of fluorescein is known to be irreversible [ Imamura and Koizumi , ].…”

Comparison of the Slip of a PDMS Melt on Weakly Adsorbing Surfaces Measured by a New Photobleaching-Based Technique

“…The diffusion causes a broadening of the profiles that could be mistaken and interpreted as a reduction of slip. Because the fluorescence signals are integrated over y and z, there is here no effect of the amplification of diffusion due to shear [21,44]. The typical root mean square width of the initial profile is σ 0 = 15 µm.…”

“…The velocimetry technique based on the observation of the evolution of a pattern drawn in a fluorescent fluid using photobleaching has proved its efficiency for fluidsolid velocity measurement [7,8,13,14,21,22,38]. The evolution of this motif when the fluid flows enable access to the slip velocity and the slip length.…”

“…This quantity has been reported to range from few to hundreds of nanometers for simple fluids or even to hundreds of micrometers for complex fluids [4,5,8,[11][12][13][14][15][16]. The question of slip at interfaces for polymer fluids, either melts or entangled solutions, has been actively studied theoretically and experimentally for the last 20 years [17,18], both for its conceptual and practical importance in many fields such as polymer assisted oil recovery [19][20][21][22], polymer extrusion [3,23,24], lubrication in industrial processes [25] or in biological systems [26]. Indeed, measuring a slip velocity or a slip length can give direct information on the friction mechanisms between the liquid and the surface, and can thus lead to controlled manipulations of the friction through specifically designed surface modifications [7,27].…”

Electroviscous resistance of nanofluidic bends

“…Typical mean velocities through nanochannels range from O(10 −2 ) mm/s [41,43] up to O(10 1 ) mm/s [9,42]. Here we choose a relatively high velocity of 5 mm/s for most of the V 0.05-50.0 [9,[40][41][42][43] simulations to determine whether convective effects play an important role in these systems. However, we will show that this is not generally the case, and that the resistance of the bend is independent of the velocity (and hence flow rate) over the range of typical velocities present in these systems.…”