C.U. Murade

Electrowetting driven optical switch and tunable aperture

We demonstrate an electrowetting based optical switch with tunable aperture. Under the influence of an electric field a non-transparent oil film can be replaced locally by a transparent water drop creating an aperture through which light can pass. Its diameter can be tuned between 0.2 and 1.2 mm by varying the driving voltage or frequency. The on and off response time of the switch is in the order of 2 and 120 ms respectively. Finally we demonstrate an array of switchable apertures that can be tuned independently or simultaneously.

Interaction of Oxazole Yellow Dyes with DNA Studied with Hybrid Optical Tweezers and Fluorescence Microscopy

We have integrated single molecule fluorescence microscopy imaging into an optical tweezers set-up and studied the force extension behavior of individual DNA molecules in the presence of various YOYO-1 and YO-PRO-1 concentrations. The fluorescence modality was used to record fluorescent images during the stretching and relaxation cycle. Force extension curves recorded in the presence of either dye did not show the overstretching transition that is characteristic for bare DNA. Using the modified wormlike chain model to curve-fit the force extension data revealed a contour length increase of 6% and 30%, respectively, in the presence of YO-PRO-1 and YOYO-1 at 100 nM. The fluorescence images recorded simultaneously showed that the number of bound dye molecules increased as the DNA molecule was stretched and decreased again as the force on the complex was lowered. The binding constants and binding site sizes for YO-PRO-1 and YOYO-1 were determined as a function of the force. The rate of YO-PRO-1 binding and unbinding was found to be 2 orders of magnitude larger than that for YOYO-1. A kinetic model is proposed to explain this observation.

Electrically assisted drop sliding on inclined planes

We demonstrate that electrowetting using alternating current (ac) voltage can be used to overcome pinning of small drops due to omnipresent heterogeneities on solid surfaces. By balancing contact angle hysteresis with gravity on inclined planes, we find that the critical electrowetting number for mobilizing drops is consistent with the voltage-dependent reduction in contact angle hysteresis in ac electrowetting. Moreover, the terminal velocity of sliding drops under ac electrowetting is found to increase linearly with the electrowetting number. Based on this effect, we present a prototype of a wiper-free windscreen.We demonstrate that electrowetting using alternating current (ac) voltage can be used to overcome pinning of small drops due to omnipresent heterogeneities on solid surfaces. By balancing contact angle hysteresis with gravity on inclined planes, we find that the critical electrowetting number for mobilizing drops is consistent with the voltage-dependent reduction in contact angle hysteresis in ac electrowetting. Moreover, the terminal velocity of sliding drops under ac electrowetting is found to increase linearly with the electrowetting number. Based on this effect, we present a prototype of a wiper-free windscreen.

Optofluidic lens with tunable focal length and asphericity

Adaptive micro-lenses enable the design of very compact optical systems with tunable imaging properties. Conventional adaptive micro-lenses suffer from substantial spherical aberration that compromises the optical performance of the system. Here, we introduce a novel concept of liquid micro-lenses with superior imaging performance that allows for simultaneous and independent tuning of both focal length and asphericity. This is achieved by varying both hydrostatic pressures and electric fields to control the shape of the refracting interface between an electrically conductive lens fluid and a non-conductive ambient fluid. Continuous variation from spherical interfaces at zero electric field to hyperbolic ones with variable ellipticity for finite fields gives access to lenses with positive, zero, and negative spherical aberration (while the focal length can be tuned via the hydrostatic pressure).

High speed adaptive liquid microlens array

Liquid microlenses are attractive for adaptive optics because they offer the potential for both high speed actuation and parallelization into large arrays. Yet, in conventional designs, resonances of the liquid and the complexity of driving mechanisms and/or the device architecture have hampered a successful integration of both aspects. Here we present an array of up to 100 microlenses with synchronous modulation of the focal length at frequencies beyond 1 kHz using electrowetting. Our novel concept combines pinned contact lines at the edge of each microlens with an electrowetting controlled regulation of the pressure that actuates all microlenses in parallel. This design enables the development of various shapes of microlenses. The design presented here has potential applications in rapid parallel optical switches, artificial compound eye and three dimensional imaging.

Force spectroscopy and fluorescence microscopy of dsDNA–YOYO-1 complexes: implications for the structure of dsDNA in the overstretching region

When individual dsDNA molecules are stretched beyond their B-form contour length, they reveal a structural transition in which the molecule extends 1.7 times its contour length. The nature of this transition is still a subject of debate. In the first model, the DNA helix unwinds and combined with the tilting of the base pairs (which remain intact), results in a stretched form of DNA (also known as S-DNA). In the second model the base pairs break resulting effectively in two single-strands, which is referred to as force-induced melting. Here a combination of optical tweezers force spectroscopy with fluorescence microscopy was used to study the structure of dsDNA in the overstretching regime. When dsDNA was stretched in the presence of 10 nM YOYO-1 an initial increase in total fluorescence intensity of the dye–DNA complex was observed and at an extension where the dsDNA started to overstretch the fluorescence intensity leveled off and ultimately decreased when stretched further into the overstretching region. Simultaneous force spectroscopy and fluorescence polarization microscopy revealed that the orientation of dye molecules did not change significantly in the overstretching region (78.0°± 3.2°). These results presented here clearly suggest that, the structure of overstretched dsDNA can be explained accurately by force induced melting.

A microfluidic platform for on-demand formation and merging of microdroplets using electric control

We discuss a microfluidic system in which (programmable) local electric fields originating from embedded and protected electrodes are used to control the formation and merging of droplets in a microchannel. The creation of droplets-on-demand (DOD) is implemented using the principle of electrowetting. Combined with hydrodynamic control, the droplet size and formation frequency can be varied independently. Using two synchronized DOD injectors, merging-on-demand (MOD) is achieved via electrocoalescence. The efficiency of MOD is 98% based on hundreds of observations. These two functionalities can be activated independently.

Single-Molecule DNA Force Spectroscopy to Probe Interactions with the Tri-Peptide Lys-Trp-Lys

Interactions between nucleotides and proteins are essential in the life cycle of cells. Specific physico-chemical properties of the system give rise to diverse modes of interaction. Coulomb interactions between charged residues on either macromolecule provide an important contribution to the interaction potential. For uncharged residues, the interaction potential is dominated by van der Waals interactions. A special form of van der Waals interactions is intercalation, which can be understood as a stacking interaction between hydrophobic aromatic residues to avoid exposure to polar solvents. A well-known example of stacking is provided by DNA. A helical motif stacks the base molecules and stabilizes the structure by base–base interactions and shielding from the solvent. The aromatic amino acids in proteins have also been noted as structural elements with an ability to intercalate. In the helix-destabilizing DNA-binding proteins gp32 of bacteriophage T4 and gp5 of bacteriophage M13, repeats of tyrosine and/or tryptophan residues have been suggested to exemplify the importance of intercalation in DNA–protein interactions. Recent studies with optical tweezers have shown that single-molecule DNA experiments in the presence of intercalating molecules enable a study of the “molecular force” exerted while molecules intercalate. Structural parameters of the DNA influenced by the interaction can be directly obtained, namely changes in contour and persistence lengths, binding constant and binding site size as a function of applied force. The binding affinity of intercalating dye molecules (ethidium bromide, YO-1 and YOYO-1) is usually quite large, while the binding affinity of model-peptide systems containing an intercalating aromatic amino acid tryptophan or tyrosine is much smaller. Based on our work on DNA–dye molecule interactions, we investigate herein the interaction of single-molecule DNA with the tripeptide LysTrp-Lys using optical tweezers force spectroscopy. In pioneering work by H l ne and Brun using fluorescence spectroscopy, a two-step binding mechanism [Eq. (1)] based on the observed increase in binding constant as a function of nucleotide concentration has been proposed [Eq. (1)]: