Microfluidics - PIV Micro scale velocity measurements

Dantec Dynamics MicroPIV system, here with a laser as the illumination source

Introduction Particle Image Velocimetry (PIV) is now a mature technique with more than 25 years of application history in fluid mechanics. MicroPIV is one variation of this technique that describes planar two-component velocity field measurements with micron resolution.

With the advances in research in medicine, molecular biology and genetics, investigation of fluid flows on a micro scale has received increased focus. MicroPIV systems became quite popular, because they are proven robust tools in investigating biological flows, validation and test of MEMS and lab-on-a-chip devices.

The applications include: flow focusing and separation; cell detection and manipulation; assay implementation; DNA purification; protein crystallisation etc. MicroPIV systems are used in two-phase flow microsystems: micro-mixers, droplet and bubble dispensers.

Applications

• Experiments simulating biological flows

• Validation and test of flows in MEMS

• Design and development of lab-on-a-chip devices

• Two-phase flow microsystems (droplet or bubble dispensers)

• Mixing enhancement studies using magnetic or electrokinetic forces

• Validation of slip flow condition in gas flows

Features

• Integrated solution with safe delivery of laser light

• Optional integrated double-pulsed LED illumination

• Optics/seeding optimised for maximum SNR

• Measurements at high optical magnification: up to 100x

• Fluorescent seeding particles: down to 100nm • Modular system: easy upgrade to/from conventional

PIV or LIF systems • Time resolved measurements: up to 10kHz

Applications are not limited to liquid or two-phase flows; as microPIV systems are used in validation of slip-flow boundary condition in gas flows in micro-channels.

Description With more than 65 years of experience in developing diagnostic tools for fluid mechanics, Dantec Dynamics has developed a dedicated MicroPIV system for performing velocity measurements in microfluidic devices. The turnkey system is optimised for maximum light transmission and therefore for optimised signal to noise ratio (SNR).

The components of the system are designed to make PIV measurements safe and user-friendly, so that the researcher can focus on the experiment and not on the equipment or laser safety.

Microfluidics – PIV Product Information

Illumination

In conventional PIV, a light sheet defines the measurement plane, the camera field-of-view (FOV) defines the measurement area and the light sheet thickness defines the thickness of the measurement volume. For MicroPIV the illumination mode is often volume illumination, as even a thin light sheet often has a thickness similar to the channel dimension. The microscope objective’s depth-of-field determines the thickness of the measurement volume.

Illustration of light sheet and volume illumination.

Laser illumination and safety

Many microfludic measurements require pulsed laser illumination to “freeze” the particle motion in a small FOV. The MicroPIV system has unique safety features:

• A flexible optical guide ensures safe delivery of laser light into the microscope

• Special laser and microscope optics block emission of invisible harmonics

• A cover plate over the translation stage reduces the amount of light above the microscope

• An interlock system ensures that the laser light is never in the eyepieces of the microscope

Standard laser safety features of the MicroPIV system: liquid light guide, laser interlock and safety cover.

Pulsed LED illumination - Microstrobe

For slow microfludic flows in transparent samples with less than 100mm/s velocity, the Microstobe can be used. The compact, dedicated, low cost, pulsed LED illumination eliminates all the laser safety requirements ensuring a completely safe operating environment for the researcher.

Transmission microscopy with the Microstrobe illumination

Microscope

The HiPerformance Microscope is an inverted fluorescense microscope that is optimized for microfluidic applications. The microscope can be delivered with a manual XY stage or a motorized XYZ stage. Please refer to the separate HiPerformance Inverted Microscope Product Information for details. On request Dantec Dynamics can implement laser delivery, including a laser safety kit, for other microscopes.

Filter cube

In a fluorescent microscope, both illumination and imaging take place through the microscope objective. An optical filter cube reflects the green light from the laser and transmits the broadband fluorescent emission to the camera detector.

Schematic of optical paths in fluorescent microscopy.

The performance of filter cube is optimised for maximum signal to noise ratio (SNR); by selecting the right optical components and seeding particles that produce, reflect and transmit the useful wavelengths and block the unwanted harmonics involved in the process.

Microfluidics – PIV Product Information

Objectives

High quality objectives are essential for recording the particle positions accurately in PIV images. In volume illumination, the camera FOV defines the measurement area, but it is the numerical aperture of the microscope objective that defines the flatness and the thickness of the measurement plane.

Objectives with various magnifications

The microscope objectives are designed to withstand the high-energy laser pulses in the measurement volume. Finally, the green laser emission and broadband fluorescense emission should be transmitted with minimum loss through the objective.

For these reasons, only high-quality dry objectives are used in Dantec Dynamics MicroPIV systems, with the following technical specifications:

M

Numerical aperture

NA

Working distance

[mm]

Cover glass correction

[mm]

Depth of field [µm]

×5 0.12 14 – 18.5

×10 0.25 12 – 5.5

×20 0.40 6.9 0 – 2 1.7

×40 0.60 3.3 – 1.9 0 – 2 0.9

×63 0.70 2.6 0.1 – 1.3 0.6

×100 0.75 4.7 – 0.5

Camera

When it comes to camera selection, Dantec Dynamics is the most flexible choice. With more than 25 different camera models integrated in Dynamic Studio, it is easy to pick a suitable camera for the experiment’s spatial resolution, pixel sensitivity and temporal resolution requirements: The camera range includes CCD, CMOS and sCMOS detectors, from VGA to 29Mpix resolution, up to 70% absolute quantum efficiency (QE) and up to 16000 frames per second. Many camera mounts are available to adapt the FOV to the detector size.

HiSense MkII camera featuring 70% QE

Seeding particles

Seeding is one of the most important elements for obtaining successful MicroPIV results. First of all, the seeding particles provide a strong fluorescent signal. Second, the excitation and emission wavelengths of seeding particles, are compatible with the rest of the optical system, which is designed to maximise the signal to noise ratio. Finally, diameters down to 100nm are available in order to address the ever-increasing high spatial resolution requirements in microfluidics community.

Fluorescent seeding particles

System Computer

The MicroPIV system includes a Data Acquision and Control System, which is preconfigured, tested and ready to use. All necessary hardware/software installation is performed before delivery.

The system controller is from an international manufacturer with worldwide presence.

Software

The MicroPIV system includes a full version of Dynamic Studio, which supports: • more than 25 camera models • any illumination device through Light Source Wizard • distributed data acquisition and distributed data storage • easy set-up with plug-and-play connection of cameras,

timer devices and other hardware • Image Processing Library (IPL) – a comprehensive

collection of advanced image processing functions

Microfluidics – PIV Product Information

IPL helps reveal the flow field even in poor-contrast images

• Proper Orthogonal Decomposition (POD) – advanced energy based flow decomposition

• Matlab link to perform end-user specified analysis routines scripted in Matlab

• advanced 2D PIV algorithms: adaptive cross-correlation, adaptive PIV and Least Squares Matching (LSM)

2D LSM results for a counter rotating vortex system. Vector map and vorticity contours are calculated simultaneously without pre-processing and are shown without subsequent smoothing.

Adaptive PIV iteratively optimizes the size and shape of each interrogation window to local flow gradients and seeding concentration.

• advanced 2D PTV algorithms based on 3-, and

4-frame tracking

2D PTV results of a toroidal vortex. TR particle tracking is a powerful tool for computing Lagrangian velocity fields in sparsely seeded flows, often the case for MicroPIV.

• calculation of vector derivatives like vorticity, streamlines, and λ2 parameter

• advanced graphical representation of the measured parameters without requiring additional third party software such as Tecplot

• Stability analysis using Oscillating Pattern Decomposition (OPD)

Stability analysis of a pulsed jet using OPD. Left: spatial representation of the least stable mode, Right: decay time of the OPD modes plotted against associated frequencies.

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Additional information For additional information and ordering please contact your Dantec Dynamics representative.

Dantec Dynamics undertakes a continuous and intensive product development programme to ensure that its instruments perform to the highest technical standards. As a result the specifications in this document are subject to change without notice.

Publication No.: pi_103_v10