Laser Induced Fluorescence Photobleaching Anemometer Research Articles

Numerical Simulation of the Photobleaching Process in Laser-Induced Fluorescence Photobleaching Anemometer

At present, a novel flow diagnostic technique for micro/nanofluidics velocity measurement—laser-induced fluorescence photobleaching anemometer (LIFPA)—has been developed and successfully applied in broad areas, e.g., electrokinetic turbulence in micromixers and AC electroosmotic flow. Nevertheless, in previous investigations, to qualitatively reveal the dynamics of the photobleaching process of LIFPA, an approximation of uniform laser distribution was applied. This is different from the actual condition where the laser power density distribution is normally Gaussian. In this investigation, we numerically studied the photobleaching process of fluorescent dye in the laser focus region, according to the convection–diffusion reaction equation. The profiles of effective dye concentration and fluorescence were elucidated. The relationship between the commonly used photobleaching time constant obtained by experiments and the photochemical reaction coefficient is revealed. With the established model, we further discuss the effective spatial resolution of LIFPA and study the influence of the detection region of fluorescence on the performance of the LIFPA system. It is found that at sufficiently high excitation laser power density, LIFPA can even achieve a super-resolution that breaks the limit of optical diffraction. We hope the current investigation can reveal the photobleaching process of fluorescent dye under high laser power density illumination, to enhance our understanding of fluorescent dynamics and photochemistry and develop more powerful photobleaching-related flow diagnostic techniques.

Transition from periodic to chaotic AC electroosmotic flows near electric double layer

AbstractElectroosmotic flows (EOFs) on insulated interfacial surface commonly exists as interfacial flows. Previously theoretical studies indicate that EOFs of Newtonian fluids on the insulated interfacial surface are steady in microchannels with symmetric zeta potentials (Suresh and Homsy, Physics of Fluids, 2004, 16, 2,349). Restricted by flow diagnostic methods in microfluidics, few velocity measurements of instantaneous EOFs have been reported, and the existence of unsteady EOFs on the insulated surface remains unclear. In this investigation, the velocity fluctuations of EOFs generated under AC electric field (named as ACFEOF) overlapped on a steady pressure‐driven flow are measured by laser induced fluorescence photobleaching anemometer, at the diffuse electric double layer (EDL) on the bottom wall far from electrodes. Chaotic velocity fluctuations according to unsteady ACFEOF has been, for the first time, observed. Stokes number (St) and electrical Reynolds number (ReE) related to oscillation and electro‐inertial effect are suggested to control chaotic ACFEOF.

Asymmetric temporal variation of oscillating AC electroosmosis with a steady pressure-driven flow

In most electrokinetic flows in micro-/nanofluidics, electroosmotic flow (EOF) is always a fundamental phenomenon. When an external AC electric field is applied, there will be a transient EOF velocity, which is crucial for understanding the mechanism of the unsteady AC EOF. However, the transient flow velocity cannot be experimentally measured with sufficiently and simultaneously temporal and spatial resolution. Here we report that under a strong AC electric field, an asymmetric temporal variation of oscillating EOF (OEOF) can be generated near the electric double layer (EDL) on the bottom wall of a microchannel. The flow is measured by laser-induced fluorescence photobleaching anemometer (LIFPA). Both the flow velocity of the OEOF and the linearly related internal electric field in the microchannel are theoretically governed by an inviscid Burgers’ equations with an external forcing term and controlled by two coefficients which represent the magnitudes of linear ( $${Z}_{l}$$ ) and nonlinear ( $${Z}_{nl}$$ ) terms. These two coefficients are separately determined by five dimensionless parameters, including Stokes number ( $${St}$$ ), aspect ratio ( $$\gamma$$ ), Schmidt number ( $${Sc}$$ ), Peclet number of mean flow velocity ( $${P}_{e, U}$$ ) and Peclet number of electrophoresis ( $${P}_{e, {EP}}$$ ). In this work, the influence of $${P}_{e, U}$$ and $${P}_{e, {EP}}$$ which are tightly related to the local mean flow velocity and the amplitude of AC electric field is investigated. When the electric field is low, $${P}_{e, U}\gg {P}_{e,{EP}}$$ and accordingly $${Z}_{l}\gg {Z}_{nl}$$ , a temporal symmetry of the time series of velocity is observed. However, when the electric field is high, $${P}_{e, U}\ll {P}_{e, {EP}}$$ and $${Z}_{l}\ll {Z}_{nl}$$ , then the periodicity and temporal symmetry of OEOF can be significantly broken and a temporal asymmetry of the time series of velocity is resulted in, especially under the square wave external electric field. We also find increased mean flow velocity and saturation of velocity fluctuations with the increasing electric field. This work provides a new methodology to study unsteady electrokinetic flows.

Parametric study of the emission spectra and photobleaching time constants of a fluorescent dye in laser induced fluorescence photobleaching anemometer (LIFPA) applications

As a novel velocity measurement technique for micro/nanofluidics applications, the laser-induced fluorescence photobleaching anemometer (LIFPA) has been proven to have simultaneously high spatial and temporal resolutions. Its temporal resolution, which is crucial for investigations of transient flows, is directly determined by the photobleaching time constant. In this study, the factors, e.g., fluorescent excitation and emission spectra and the photobleaching time constant that may affect LIFPA measurements, were investigated using the coumarin 102 fluorescent dye at different concentrations (10–300 μM) in a confocal-microscope-based system. We report that a blue shift of the emission spectrum in the range of 430–515 nm can be observed during photobleaching attributed to the photochemical reactions between coumarin 102 molecules. Meanwhile, different photobleaching time constants are also observed in different bands of the emission spectrum. All of the photobleaching time constants are below 3 μs, which is nearly an order smaller than the previously reported values. Among all of the spectral bands, the bands at 470–492 nm can provide a higher fluorescent intensity and better accuracy for velocity measurements. The bands at 430 and 460 nm can provide a higher temporal resolution.

Study of Oscillating Electroosmotic Flows with High Temporal and Spatial Resolution.

Near-wall velocity of oscillating electroosmotic flow (OEOF) driven by an AC electric field has been investigated using a laser-induced fluorescence photobleaching anemometer (LIFPA). For the first time, an up to 3 kHz velocity response of OEOF has been successfully measured experimentally, even though the oscillating velocity is as low as 600 nm/s. It is found that the oscillating velocity decays with the forcing frequency ff as ff-0.66. In the investigated range of electric field intensity (EA), below 1 kHz, the linear relation between oscillating velocity and EA is also observed. Because the oscillating velocity at high frequency is very small, the contribution of noise to velocity measurement is significant, and it is discussed in this manuscript. The investigation reveals the instantaneous response of OEOF to the temporal change of electric fields, which exists in almost all AC electrokinetic flows. Furthermore, the experimental observations are important for designing OEOF-based micro/nanofluidics systems.

Microelectrokinetic turbulence in microfluidics at low Reynolds number

There is commonly no turbulence in microfluidics, and the flows are believed to be either laminar or chaotic, since Reynolds number (Re) in microflows is usually on the order of unity or lower. However, we recently demonstrated that it is possible to achieve turbulence with low Re (based on the measured flow velocity and the width of the channel entrance) when a pressure-driven flow is electrokinetically forced in a quasi T-microchannel. To be able to measure high frequency velocity fluctuations in microchannels, a velocimeter with submicrometer spatial resolution and microsecond temporal resolution, called a laser-induced fluorescence photobleaching anemometer, is developed. Here we characterize the microelectrokinetic turbulence and observe some typical and important features of high Re flows, such as Kolmogorov -5/3 spectrum of velocity fluctuation, which usually can be realized only at very high Re in macroturbulent flows.

Measurement of velocity fluctuations in microfluidics with simultaneously ultrahigh spatial and temporal resolution

Although unsteady and electrokinetic flows are widely used in microfluidics, there is unfortunately no velocimeter today that can measure the random velocity fluctuation at high temporal and spatial resolution simultaneously in microfluidics. Here we, for the first time, theoretically study the temporal resolution of laser induced fluorescence photobleaching anemometer (LIFPA) and experimentally verify that LIFPA can have simultaneously ultrahigh temporal \(({\sim } 4\,\upmu \hbox {s})\) and spatial \(({\sim }203\,\hbox {nm})\) resolution and can measure velocity fluctuation up to at least 2 kHz, whose corresponding wave number is about \(6\times 10^6\,{/}\hbox {m}\) in an electrokinetically forced unsteady flow in microfluidics.

Corrections on LIFPA velocity measurements in microchannel with moderate velocity fluctuations

Laser-induced fluorescence photobleaching anemometer (LIFPA) has been developed in order to measure velocity fluctuations of the unsteady micro electrokinetic turbulent flows in microfluidics. The statistical errors of LIFPA measurement, because of 3-D flows and Taylor’s hypothesis (compared with local Taylor’s hypothesis Pinton and Labbe in J Phys II 4:1461–1468, 1994), are theoretically estimated and compared to hot-wire anemometer (HWA) measurement that is used for conventional turbulence measurement. The correction factor in the direction parallel to the laser beam is estimated, and the influence of directional correction factors of LIFPA is also investigated. It is found that in our investigation, the error due to Taylor’s hypothesis is negligible. The influence of 3-D flows on the first derivative variance of velocity fluctuations in LIFPA is smaller than that in HWA measurement.

Study on factors enhancing photobleaching effect of fluorescent dye

With unique capability of biomedical diagnosis and chemical analysis, microfluidics has received worldwide applications in various fields. Multiple methods have been proposed to measure velocity profile of flow in microchannels, among which the laser induced fluorescence photobleaching anemometer (LIFPA) provides great performance with not only high spatial resolution but also superior temporal resolution compared with former systems. The measurement sensitivity and temporal resolution of LIFPA system is largely determined by the speed of fluorescence photobleaching effect, meaning that a half-life decay time as short as possible is required to achieve superior temporal resolution. In this paper, several experiments are done to investigate the influence over the half-life decay time by several factors, including incident laser power, dye concentration, buffer concentration and fluidic pH value. Based on the experimental results, a certain dye solution is specially designed, which demonstrates an extremely fast photobleaching effect with a half-life decay time of about 55μs. Fluorescent dye solution with such a fast photobleaching effect is potentially applicable in the analysis of transient flows, such as the rise time of electroosmotic flow, where high temporal resolution is required.

Ultrafast measurement of transient electroosmotic flow in microfluidics

We present a non-intrusive molecular dye based method, i.e., laser-induced fluorescence photobleaching anemometer (LIFPA), to significantly increase temporal resolution (TR) for velocity measurement of fast transient electrokinetic flows. To our knowledge, the TR has been for the first time achieved to 5–10 μs, about 100 times better than that published from state-of-the-art micro particle image velocimetry (μPIV), which is currently the most widely used velocimetry in the microfluidics community. The new method provides us with new opportunities to study experimentally the fundamental phenomena of unsteady electrokinetics (EK) and to validate relevant theoretical models. One application of the new method is demonstrated by measuring the rise time of DC electroosmotic flows (EOFs) in a microcapillary of 10 μm in diameter.

A novel far-field nanoscopic velocimetry for nanofluidics

For the first time we have been able to measure the flow velocity profile for nanofluidics with a spatial resolution better than 70 nm. Due to the diffraction resolution barrier, traditional optical methods have so far failed in measuring the velocity profile in a nanocapillary or a closed nanochannel without an opened sidewall. A novel optical point measurement method is presented which applies stimulated emission depletion (STED) microscopy to laser induced fluorescence photobleaching anemometer (LIFPA) techniques to measure flow velocity. Herein we demonstrate this far-field nanoscopic velocimetry method by measuring the velocity profile in a nanocapillary with an inner diameter of 360 nm. The closest measuring point to the wall is about 35 nm. This method opens up a new class of functional measuring techniques for nanofluidics and for nanoscale flows from the wall.

Study of the Rise Time in Electroosmotic Flow within a Microcapillary

For the first time with a temporal resolution higher than 100 micros, we have successfully measured the risetime of electroosmotic flow (EOF) in a microcapillary using recently developed laser induced fluorescence photobleaching anemometer (LIFPA). Although there are some theoretical estimations in literature about the risetime in microcapillaries, to the best of our knowledge, this has never been experimentally validated in a microcapillary with inner diameter less than 100 microm. The LIFPA with high temporal and spatial resolution described in this paper is capable of measuring risetime distribution radially in a cylindrical microcapillary tube. The experimental results show that the risetime under a pulsed electric field decreases with the reduction of the inner diameter of a microcapillary, and the risetime also increases from the wall to the axis for a given microcapillary. These are qualitatively similar to the theoretical prediction. In addition, the initial pressure driven flow does not appear to affect risetime.

Measuring flow velocity distribution in microchannels using molecular tracers

We demonstrate the capability of a molecular tracer based laser induced fluorescence photobleaching anemometer for measuring fluid velocity profile in microfluidics. To validate the feasibility and accuracy of this measurement system, the velocity profiles of the cylindrical and rectangular microchannels are measured, respectively. We compare our experimental results with theoretical prediction. The theoretical prediction shows a good consistency with practical measurement results. The spatial resolutions are analyzed and validated. Finally, the error source of the presented system is analyzed, which makes it possible to compensate some error quantitatively and guides the application of this system.

Photobleaching‐based flow measurement in a commercial capillary electrophoresis chip instrument

For microfluidic analytical instruments, a facile, fast, and accurate instrument test is highly demanded. The test includes the quantitative verification of the relationship between pressure drop and flow velocity for the hydrodynamic pump, between the electric voltage and electroosmotic flow (EOF) for the high-voltage supply, and the chip quality. The key point for the test is the measurement of the flow velocity. However, most currently available velocimetries cannot be directly used without any instrumental modification or adding extra instruments. We applied a recently developed Laser Induced Fluorescence Photobleaching Anemometer (LIFPA) for the instrument test through measuring fluid flow velocity in a microfluidic instrument with optical measurement without any modification and extra instrument. We have successfully used the method to test Caliper HTS 250 System from Caliper Life Sciences (Hopkinton, MA) with its own light source and detector. The experimental result demonstrates that this single-point method of measuring flow velocity can be easily used for accurate test of a microfluidic instrument in less than 10 min at extremely low cost without any modification and extra instrument.

Laser induced fluorescence photobleaching anemometer for microfluidic devices

We have developed a novel, non-intrusive fluid velocity measurement method based on photobleaching of a fluorescent dye for microfluidic devices. The residence time of the fluorescent dye in a laser beam depends on the flow velocity and approximately corresponds to the decaying time of the photobleaching of the dye in the laser beam. The residence time is inversely proportional to the flow velocity. The fluorescence intensity increases with the flow velocity due to the decrease of the residence time. A calibration curve between fluorescence intensity and known flow velocity should be obtained first. The calibration relationship is then used to calculate the flow velocity directly from the measured fluorescence intensity signal. The new method can measure the velocity very quickly and is easy to use. It is demonstrated for both pressure driven flow and electroosmotic flow.