Vincent Thomy

Integrated microfluidics based on multi-layered SU-8 for mass spectrometry analysis

We present a design for integrated lab-on-chip microsystems dedicated to mass spectrometry analysis based on the fabrication of watertight microchannels for the circulation of liquids. In this paper, we demonstrate how to fabricate complete polymer microchannels using the negative photoresist SU-8 which has the advantage of being compatible with protein analysis by mass spectrometry. Our method of fabrication requires novel technological steps involving SU-8 multi-layer processing, improved SU-8 adhesion and the use of SU-8 wafer bonding for the watertight closing of the microchannels with a Pyrex wafer. This technique also encompasses the design of various microfluidic elements such as tapered recesses for the housing of capillary tubes allowing the connection of the channels to external systems. Following this, the capillary tubes were used to test the hydrodynamic behaviour of the channels and consequently the efficiency of our technological process in achieving fully watertight structures within our flow rate and pressure specifications.

Engineering Sticky Superomniphobic Surfaces on Transparent and Flexible PDMS Substrate

Following the achievement of superhydrophobicity which prevents water adhesion on a surface, superomniphobicity extends this high repellency property to a wide range of liquids, including oils, solvents, and other low surface energy liquids. Recent theoretical approaches have yield to specific microstructures design criterion to achieve such surfaces, leading to superomniphobic structured silicon substrate. To transfer this technology on a flexible substrate, we use a polydimethylsiloxane (PDMS) molding process followed by surface chemical modification. It results in so-called sticky superomniphobic surfaces, exhibiting large apparent contact angles (>150°) along with large contact angle hysteresis (>10°). We then focus on the modified Cassie equation, considering the 1D aspect of wetting, to explain the behavior of droplets on these surfaces and compare experimental data to previous works to confirm the validity of this model.

Zipping Effect on Omniphobic Surfaces for Controlled Deposition of Minute Amounts of Fluid or Colloids

When a drop sits on a highly liquid-repellent surface (super-hydrophobic or super-omniphobic) made of periodic micrometer-sized posts, its contact-line can recede with very weak mechanical retention providing that the liquid stays on top of the microsized posts. Occurring in both sliding and evaporation processes, the achievement of low-contact-angle hysteresis (low retention) is required for discrete microfluidic applications involving liquid motion or self-cleaning; however, careful examination shows that during receding, a minute amount of liquid is left on top of the posts lying at the receding edge of the drop. For the first time, the heterogeneities of these deposits along the drop-receding contact-line are underlined. Both nonvolatile liquid and particle-laden water are used to quantitatively characterize what rules the volume distribution of deposited liquid. The experiments suggest that the dynamics of the liquid de-pinning cascade is likely to select the volume left on a specific post, involving the pinch-off and detachment of a liquid bridge. In an applied prospective, this phenomenon dismisses such surfaces for self-cleaning purposes, but offers an original way to deposit controlled amounts of liquid and (bio)-particles at well-targeted locations.

Sliding Droplets on Superomniphobic Zinc Oxide Nanostructures

This study reports on liquid-repellency of zinc oxide nanostructures (ZnO NS). The ZnO NS are synthesized by an easy and fast chemical bath deposition technique. Three different nanostructured surfaces consisting of nanorods, flowers, and particles are prepared, depending on the deposition time and the presence of ethanolamine in the reaction mixture. Chemical functionalization of the ZnO NS with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS) in liquid (PFTS L) and vapor phase (PFTS V) or through octafluorobutane (C(4)F(8)) plasma deposition led to the formation of superomniphobic surfaces. A comprehensive characterization of the wetting properties (static contact angle and contact angle hysteresis) has been performed using liquids composed of deionized water and various concentrations of ethanol (surface tension between 35 and 72.6 mN/m). Depending on the nanostructures morphology, coating nature and liquid employed, high static apparent contact angles θ ≈ 150-160°, and low contact angle hysteresis Δθ ≈ 0° are obtained. The different ZnO NS are characterized using scanning electron microscopy (SEM) and contact angle measurements. The results reported in this work permit preparation of sliding omniphobic surfaces using a simple and low cost technique.

Contact angle hysteresis origins: Investigation on super-omniphobic surfaces

Contact angle hysteresis of liquid droplets is investigated on sticky and flexible super-omniphobic surfaces made up of PDMS–Si3N4 microstructures. Up to now, extensive studies have been focusing on the relation between hysteresis and surface properties such as roughness or defect density. However, little attention has been paid to the dependence of hysteresis with respect to the liquid surface tension. In this work, advancing and receding apparent contact angles are measured on surfaces displaying 4 different defect densities with 10 water–ethanol mixtures (surface energy ranging from 72 to 21.7 mN m−1). While advancing angles are found to be constant whatever the defect density and the liquid surface energy, receding angles exhibit more complex variations. Surprisingly, we noticed a saturation of this receding angle at low surface energy. In order to explain this phenomenon, we address the receding contact line distortion from the point of view of micro capillary bridges formation and breakage. The model is supported by fine SEM observation of the local deformation and offers a new perspective to explain the underlying mechanism of the saturation phenomenon.

Reversible electrowetting on superhydrophobic double-nanotextured surfaces.

The paper reports on wetting, electrowetting (EW), and systematic contact angle hysteresis measurements after electrowetting of superhydrophobic silicon nanowire surfaces (NWs). The surfaces consist of C4F8-coated silicon nanowires grown on Si/SiO2 substrate. Different surfaces modulating (i) the dielectric layer thickness and (ii) the nanotexturation were investigated in this study. It was found that the superhydrophobic NWs display different EW behaviors according to their double nanotexturation with varying droplet impalement levels. Some surfaces exhibited a total reversibility to EW with no impalement (contact angle variation of 35+/-2 degrees at 190 VTRMS with deionized water), whereas other surfaces showed nonreversible behavior to EW with partial droplet impalement. A scenario is proposed to explain the unique properties of these surfaces.

High pressure-resistant SU-8 microchannels for monolithic porous structure integration

Integrated lab-on-chip (LOC) microsystems dedicated to proteomic analysis require specific pretreatment steps such as protein trypsic digestion, concentration, desalting or separation of biological samples. These steps can be achieved thanks to porous monolithic polymers. This paper deals with the integration of such a polymer into SU-8 microchannels by using a multi-material technology (SU-8, Pyrex and silicon). A solution for the fabrication of complete polymer microchannels which are high pressure- and solvents-resistant is proposed. This technique uses the negative photoresist SU-8 which is compatible with the protein analysis performed here. Our process requires a novel technological step using a silane coupling agent. This modification of the SU-8/Pyrex interface leads to the fabrication of a 100 µm × 160 µm section microchannel (length of 3 cm), closed with a Pyrex® lid by SU-8 bonding resistant to 80 bar. An improvement of the SU-8/monolithic structure is also demonstrated thanks to a specific treatment of the polymer enabling good anchoring of the monolith in the microchannels, and the pressure-resistance tests were also achieved with the monolithic structure integrated in the microchannels. A digestion step of a protein sample of benzoylarginine ethyl ester in a SU-8 microchannel was achieved after the functionalization of a monolith anchored in the microchannel. Analysis by UV/VIS spectroscopy of this in situ digestion has been reported.

Electro-(de)wetting on superhydrophobic surfaces.

Usually, electrowetting on superhydrophobic surfaces (EWOSS) is generated by application of an alternating current signal and often leads to droplet impalement into the structuration. To avoid this phenomenon, superhydrophobic surfaces must show robustness to high pressure. Otherwise, an external energy has to be applied to dewet the droplet from the structuration. We present, in this article, an original approach to actuate liquid droplets via a modulated EWOSS signal (MEWOSS). This technique allows the dewetting of the droplet due to periodic vibrations induced by the electrowetting actuation. In that case, it is possible to investigate a larger range of superhydrophobic surfaces under EWOSS without droplet impalement. Three different superhydrophobic surfaces, showing different degrees of impalement under EWOSS, are investigated and compared using this MEWOSS technique.

Acoustic Tracking of Cassie to Wenzel Wetting Transitions

Many applications involving superhydrophobic materials require accurate control and monitoring of wetting states and wetting transitions. Such monitoring is usually done by optical methods, which are neither versatile nor integrable. This letter presents an alternative approach based on acoustic measurements. An acoustic transducer is integrated on the back side of a superhydrophobic silicon surface on which water droplets are deposited. By analyzing the reflection of longitudinal acoustic waves at the composite liquid-solid-vapor interface, we show that it is possible to track the local evolution of the Cassie-to-Wenzel wetting transition efficiently, as induced by evaporation or the electrowetting actuation of droplets.

Inhibiting protein biofouling using graphene oxide in droplet-based microfluidic microsystems

Biofouling or adsorption of biomolecules onto surfaces in microfluidic devices limits the type of samples which can be handled. In this paper, we take advantage of the high adsorption capacity of graphene oxide (GO) for proteins as a strategy to limit biofouling, while preserving their activity for droplet-based lab-on-chip applications.