Daniël Wijnperle

Electrokinetic label-free screening chip: a marriage of multiplexing and high throughput analysis using surface plasmon resonance imaging

We present an electrokinetic label-free biomolecular screening chip (Glass/PDMS) to screen up to 10 samples simultaneously using surface plasmon resonance imaging (iSPR). This approach reduces the duration of an experiment when compared to conventional experimental methods. This new device offers a high degree of parallelization not only for analyte samples, but also for multiplex analyte interactions where up to 90 ligands are immobilized on the sensing surface. The proof of concept has been demonstrated with well-known biomolecular interactant pairs. The new chip can be used for high throughput screening applications and kinetics parameter extraction, simultaneously, of interactant-protein complex formation.

High-throughput sorting of drops in microfluidic chips using electric capacitance

We analyze a recently introduced approach for the sorting of aqueous drops with biological content immersed in oil, using a microfluidic chip that combines the functionality of electrowetting with the high throughput of two-phase flow microfluidics. In this electrostatic sorter, three co-planar electrodes covered by a thin dielectric layer are placed directly below the fluidic channel. Switching the potential of the central electrode creates an electrical guide that leads the drop to the desired outlet. The generated force, which deflects the drop, can be tuned via the voltage. The working principle is based on a contrast in conductivity between the drop and the continuous phase, which ensures successful operation even for drops of highly conductive biological media like phosphate buffered saline. Moreover, since the electric field does not penetrate the drop, its content is protected from electrical currents and Joule heating. A simple capacitive model allows quantitative prediction of the electrostatic forces exerted on drops. The maximum achievable sorting rate is determined by a competition between electrostatic and hydrodynamic forces. Sorting speeds up to 1200 per second are demonstrated for conductive drops of 160 pl in low viscosity oil.

Imaging local acoustic pressure in microchannels

A method for determining the spatially resolved acoustic field inside a water-filled microchannel is presented. The acoustic field, both amplitude and phase, is determined by measuring the change of the index of refraction of the water due to local pressure using stroboscopic illumination. Pressure distributions are measured for the fundamental pressure resonance in the water and two higher harmonic modes. By combining measurement at a range of excitation frequencies, a frequency map of modes is made, from which the spectral line width and Q-factor of individual resonances can be obtained.

Microfluidic valves with integrated structured elastomeric membranes for reversible fluidic entrapment and in situ channel functionalization

We report the utility of structured elastomeric membranes (SEMs) as a multifunctional microfluidic tool. These structured membranes are part of a two-layer microfluidic device, analogous to membrane valves, with the novelty that they incorporate topographical features on the roof of the fluid channel. We demonstrate that when the topographical features are recessed into the roof of the fluid channel, actuation of the membrane leads to effective confinement of fluids down to femtolitres in preformed microfluidic containers. Thus, the SEMs in this case function as fluidic traps that could be coupled to microfluidic networks for rapid and repeated flushing of solvents. Alternatively, when the topographical features on the roof protrude into the fluid channel, we demonstrate that the SEMs can be used to pattern proteins and cells in microchannels. Thus in this case, the SEMs serve as fluidic stamps for functionalizing microchannel surfaces. In addition, we show that the trap or pattern shape and size can be manipulated simply by varying the topography on the elastomeric membrane. Since SEMs, membrane valves and pumps use similar fabrication technology, we believe that SEMs can be integrated into microfluidic large-scale circuits for biotechnological applications.

Bubble formation in catalyst pores; curse or blessing?

H2O2 decomposition experiments on Pt were performed in a glass microreactor, simulating arrays of catalyst pores. The formation of bubbles inside the model nanopores was observed with an optical microscope. It was found that the bubble initiation time strongly depends on the diffusion length and the H2O2 concentration. The amount of catalyst did not have a significant effect, suggesting that the reaction is diffusion limited. Results show that bubble formation can decrease the reaction rate by physically blocking the active sites, but also can accelerate the reaction by creating a forced convective flow inside the nanochannels due to bubble migration. Similar behaviour is likely to occur in a real catalyst and thus, a smart design of the catalytic support could be used to enhance reaction rates.

Drops jumping on super-hydrophobic surfaces: controlling energy transfer by timed electric actuation

Aqueous sessile drops are launched from a super-hydrophobic surface by electric actuation in an electrowetting configuration with a voltage pulse of variable duration. We show that the jump height, i.e. the amount of energy that is transferred from surface energy to the translational degree of freedom, depends not only on the applied voltage but also in a periodic manner on the duration of the actuation pulse. Specifically, we find that the jump height for a pulse of optimized duration is almost twice as high as the one obtained upon turning off the voltage after equilibration of the drop under electrowetting. Representing the drop by a simple oscillator, we establish a relation between the eigenfrequency of the drop and the optimum actuation time required for most efficient energy conversion. From a general perspective, our experiments illustrate a generic concept how timed actuation in combination with inertia can enhance the flexibility and efficiency of drop manipulation operations.

Design of a hybrid advective-diffusive microfluidic system with ellipsometric detection for studying adsorption

Establishing and maintaining concentration gradients that are stable in space and time is critical for applications that require screening the adsorption behavior of organic or inorganic species onto solid surfaces for wide ranges of fluid compositions. In this work, we present a design of a simple and compact microfluidic device based on steady-state diffusion of the analyte, between two control channels where liquid is pumped through. The device generates a near-linear distribution of concentrations. We demonstrate this via experiments with dye solutions and comparison to finite-element numerical simulations. In a subsequent step, the device is combined with total internal reflection ellipsometry to study the adsorption of (cat)ions on silica surfaces from CsCl solutions at variable pH. Such a combined setup permits a fast determination of an adsorption isotherm. The measured optical thickness is compared to calculations from a triple layer model for the ion distribution, where surface complexation reactions of the silica are taken into account. Our results show a clear enhancement of the ion adsorption with increasing pH, which can be well described with reasonable values for the equilibrium constants of the surface reactions.