Michael Esseling

Two-dimensional dielectrophoretic particle trapping in a hybrid crystal/PDMS-system

Dielectrophoretic forces originating from highly modulated electric fields can be used to trap particles on surfaces. An all-optical way to induce such fields is the use of a photorefractive material, where the fields that modulate the refractive index are present at the surface. We present a method for two-dimensional particle alignment on an optically structured photorefractive lithium niobate crystal. The structuring is done using an amplitude-modulating spatial light modulator and laser illumination. We demonstrate trapping of uncharged graphite particles in periodic and arbitrary patterns and provide a discussion of the limitations and the necessary boundary conditions for maximum trapping efficiency. The photorefractive crystal is utilized as bottom part of a PDMS channel in order to demonstrate two-dimensional dielectrophoretic trapping in a microfluidic system.

Charge sensor and particle trap based on z-cut lithium niobate

The generation of adhesive regions on a z-cut lithium niobate crystal without an additional voltage supply is demonstrated. We show that the origin of the attractive force in the respective solvent is electrophoresis, which can selectively trap charged particles in illuminated regions. Using digital holographic microscopy to measure the space-charge field in a y-cut crystal, we demonstrate the difference between electrophoretic and dielectrophoretic particle manipulation. The suggested method enables the creation of arbitrary two-dimensional patterns, circumventing restrictions originating from the crystal asymmetry. Furthermore, it allows the discrimination between charged particles of different signs, thus acting as a charge sensor.

Holographic optical bottle beams

We present a convolution approach for the generation of optical bottle beams that combines established techniques of holographic optical trapping with hollow intensity distributions in order to manipulate absorbing particles. The versatility of our method is demonstrated by the simultaneous stable trapping of multiple particles at defined positions. Furthermore, the presented phase shaping technique allows for the dynamic manipulation of absorbing particles along arbitrary paths.

Multiplexing and switching of virtual electrodes in optoelectronic tweezers based on lithium niobate

We introduce a method for trapping and arranging microparticles in arbitrary two-dimensional patterns with high flexibility. For this purpose, optoelectronic tweezers based on lithium niobate as photoconductor are used to create virtual electrodes through modulated illumination. The evolving field gradients arrange microparticles due to dielectrophoretic (DEP) forces and enable an all-optical approach for DEP. In order to increase flexibility further, we investigate multiplexed electrode structures for in situ reconfiguration of particle arrangements. Using the all-optical erasure of previously written virtual electrodes, we demonstrate electrode switching and sequential particle trapping in a microchannel for microfluidic applications.

Multimodal biophotonic workstation for live cell analysis

A reliable description and quantification of the complex physiology and reactions of living cells requires a multimodal analysis with various measurement techniques. We have investigated the integration of different techniques into a biophotonic workstation that can provide biological researchers with these capabilities. The combination of a micromanipulation tool with three different imaging principles is accomplished in a single inverted microscope which makes the results from all the techniques directly comparable. Chinese Hamster Ovary (CHO) cells were manipulated by optical tweezers while the feedback was directly analyzed by fluorescence lifetime imaging, digital holographic microscopy and dynamic phase-contrast microscopy.

Photophoretic trampoline—Interaction of single airborne absorbing droplets with light

We present the light-induced manipulation of absorbing liquid droplets in air. Ink droplets from a printer cartridge are used to demonstrate that absorbing liquids—just like their solid counterparts—can interact with regions of high light intensity due to the photophoretic force. It is shown that droplets follow a quasi-ballistic trajectory after bouncing off a high intensity light sheet. We estimate the intensities necessary for this rebound of airborne droplets and change the droplet trajectories through a variation of the manipulating light field.

Opto-electric particle manipulation on a bismuth silicon oxide crystal

High-throughput manipulation of microparticles can be efficiently accomplished using electrokinetic effects. In this contribution, we demonstrate the two-dimensional investigation of internal space-charge fields inside a bismuth silicon oxide (BSO) crystal and their use for optically mediated particle trapping. The magnitude of the internal fields as well as the time constant for its build-up are measured by Zernike phase contrast and digital holography. The fast response time of a BSO crystal at very low light powers enables real-time generation of high electric field gradients. We demonstrate that this photoconductive material facilitates both electrophoretic and dielectrophoretic trapping of particles on an accessible surface.

T-junction droplet generator realised in lithium niobate crystals by laser ablation

Abstract A femtosecond laser at 800 nm was used to create micro-fluidic circuits on lithium niobate (LiNbO3) substrates by means of laser ablation, using different scanning velocities (100-500 μm/s) and laser pulse energies (1-20 μJ). The T-junction geometry was exploited to create on y-cut LiNbO3 crystals a droplet generator, whose microfluidic performance was characterized in a wide range of droplet generation frequencies, from few Hz to about 1 kHz.

Depth-resolved velocimetry of Hagen-Poiseuille and electro-osmotic flow using dynamic phase-contrast microscopy

We quantitatively investigate the axial imaging properties of dynamic phase-contrast microscopy, with a special focus on typical combinations of tracer particles and magnifications that are used for velocimetry analysis. We show, for the first time, that a dynamic phase-contrast microscope, which is the integration of an all-optical novelty filter in a commercially available inverted microscope, can visualize three-dimensional velocity fields with a significantly reduced optical sectioning depth. The depth of field for dynamic phase-contrast microscopy is extracted from the three-dimensional response function and compared with the respective values for incoherent bright-field illumination. These results are then used to perform a depth-resolved particle image velocimetry analysis of Hagen–Poiseuille as well as electro-osmotically actuated flows in a microchannel.

Conical Refraction Bottle Beams for Entrapment of Absorbing Droplets

Conical refraction (CR) optical bottle beams for photophoretic trapping of airborne absorbing droplets are introduced and experimentally demonstrated. CR describes the circular split-up of unpolarised light propagating along an optical axis in a biaxial crystal. The diverging and converging cones lend themselves to the construction of optical bottle beams with flexible entry points. The interaction of single inkjet droplets with an open or partly open bottle beam is shown implementing high-speed video microscopy in a dual-view configuration. Perpendicular image planes are visualized on a single camera chip to characterize the integral three-dimensional movement dynamics of droplets. We demonstrate how a partly opened optical bottle transversely confines liquid objects. Furthermore we observe and analyse transverse oscillations of absorbing droplets as they hit the inner walls and simultaneously measure both transverse and axial velocity components.