Out-of-Equilibrium Measurements of Kinetic Constants on a Biosensor.

Conventional measurements of kinetic constants currently in use are performed at equilibrium and may require large volumes, especially at a low association rate constant kon. If the measurements are made out of equilibrium, the values obtained may be biased by dilution of the sample with the flow of the running buffer. In some applications, the available sample volume can be very critical and requires the development of tools to measure kinetic constants with low volumes. In this paper, by combining an experimental, numerical and modeling approach, we propose a surface plasmon resonance-based method that relies on an out-of-equilibrium measurement using the effect of dilution by flow to its advantage. This new method should have a significant impact in biochemistry and medical research.

Self-Similar Relaxation of Confined Microfluidic Droplets.

We report an experimental study concerning the capillary relaxation of a confined liquid droplet in a microscopic channel with a rectangular cross section. The confinement leads to a droplet that is extended along the direction normal to the cross section. These droplets, found in numerous microfluidic applications, are pinched into a peanutlike shape thanks to a localized, reversible deformation of the channel. Once the channel deformation is released, the droplet relaxes back to a pluglike shape. During this relaxation, the liquid contained in the central pocket drains towards the extremities of the droplet. Modeling such viscocapillary droplet relaxation requires considering the problem as 3D due to confinement. This 3D consideration yields a scaling model incorporating dominant dissipation within the droplet menisci. As such, the self-similar droplet dynamics is fully captured.

Wettability patterning in microfluidic devices using thermally-enhanced hydrophobic recovery of PDMS.

Spatial control of wettability is key to many applications of microfluidic devices, ranging from double emulsion generation to localized cell adhesion. A number of techniques, often based on masking, have been developed to produce spatially-resolved wettability patterns at the surface of poly(dimethylsiloxane) (PDMS) elastomers. A major impediment they face is the natural hydrophobic recovery of PDMS: hydrophilized PDMS surfaces tend to return to hydrophobicity with time, mainly because of diffusion of low molecular weight silicone species to the surface. Instead of trying to avoid this phenomenon, we propose in this work to take advantage of hydrophobic recovery to modulate spatially the surface wettability of PDMS. Because temperature speeds up the rate of hydrophobic recovery, we show that space-resolved hydrophobic patterns can be produced by locally heating a plasma-hydrophilized PDMS surface with microresistors. Importantly, local wettability is quantified in microchannels using a fluorescent probe. This "thermo-patterning" technique provides a simple route to in situ wettability patterning in closed PDMS chips, without requiring further surface chemistry.

Ultrasound transmission through monodisperse 2D microfoams.

While the acoustic properties of solid foams have been abundantly characterized, sound propagation in liquid foams remains poorly understood. Recent studies have investigated the transmission of ultrasound through three-dimensional polydisperse liquid foams (Pierre et al., 2013, 2014, 2017). However, further progress requires to characterize the acoustic response of better-controlled foam structures. In this work, we study experimentally the transmission of ultrasounds through a single layer of monodisperse bubbles generated by microfluidics techniques. In such a material, we show that the sound velocity is only sensitive to the gas phase. Nevertheless, the structure of the liquid network has to be taken into account through a transfer parameter analogous to the one in a layer of porous material. Finally, we observe that the attenuation cannot be explained by thermal dissipation alone, but is compatible with viscous dissipation in the gas pores of the monolayer.

Investigating the role of a poorly soluble surfactant in a thermally driven 2D microfoam.

Foam drainage dynamics is known to be strongly affected by the nature of the surfactants stabilising the liquid/gas interface. In the present work, we consider a 2D microfoam stabilized by both soluble (sodium dodecylsulfate) and poorly soluble (dodecanol) surfactants. The drainage dynamics is driven by a thermocapillary Marangoni stress at the liquid/gas interface [V. Miralles et al., Phys. Rev. Lett., 2014, 112, 238302] and the presence of dodecanol at the interface induces interface stress acting against the applied thermocapillary stress, which slows down the drainage dynamics. We define a damping parameter that we measure as a function of the geometrical characteristics of the foam. We compare it with predictions based on the interface rheological properties of the solution.

Droplets in Microchannels: Dynamical Properties of the Lubrication Film.

We study the motion of droplets in a confined, micrometric geometry, by focusing on the lubrication film between a droplet and a wall. When capillary forces dominate, the lubrication film thickness evolves nonlinearly with the capillary number due to the viscous dissipation between the meniscus and the wall. However, this film may become thin enough (tens of nanometers) that intermolecular forces come into play and affect classical scalings. Our experiments yield highly resolved topographies of the shape of the interface and allow us to bring new insights into droplet dynamics in microfluidics. We report the novel characterization of two dynamical regimes as the capillary number increases: (i) at low capillary numbers, the film thickness is constant and set by the disjoining pressure, while (ii) above a critical capillary number, the interface behavior is well described by a viscous scenario. At a high surfactant concentration, structural effects lead to the formation of patterns on the interface, which can be used to trace the interface velocity, that yield direct confirmation of the boundary condition in the viscous regime.

Bubbles and foams in microfluidics.

Microfluidics offers great tools to produce highly-controlled dispersions of gas into liquid, from isolated bubbles to organized microfoams. Potential technological applications are manifold, from novel materials to scaffolds for tissue engineering or enhanced oil recovery. More fundamentally, microfluidics makes it possible to investigate the physics of complex systems such as foams at scales where the capillary forces become dominant, in model experiments involving few well-controlled parameters. In this context, this review does not have the ambition to detail in a comprehensive manner all the techniques and applications involving bubbles and foams in microfluidics. Rather, it focuses on particular consequences of working at the microscale, under confinement, and hopes to provide insight into the physics of such systems. The first part of this work focuses on bubbles, and more precisely on (i) bubble generation, where the confinement can suppress capillary instabilities while inertial effects may play a role, and (ii) bubble dynamics, paying special attention to the lubrication film between bubble and wall and the influence of confinement. The second part addresses the formation and dynamics of microfoams, emphasizing structural differences from macroscopic foams and the influence of the confinement.

Thermocapillary actuation by optimized resistor pattern: bubbles and droplets displacing, switching and trapping.

We report a novel method for bubble or droplet displacement, capture and switching within a bifurcation channel for applications in digital microfluidics based on the Marangoni effect, i.e. the appearance of thermocapillary tangential interface stresses stemming from local surface tension variations. The specificity of the reported actuation is that heating is provided by an optimized resistor pattern (B. Selva, J. Marchalot and M.-C. Jullien, An optimized resistor pattern for temperature gradient control in microfluidics, J. Micromech. Microeng., 2009, 19, 065002) leading to a constant temperature gradient along a microfluidic cavity. In this context, bubbles or droplets to be actuated entail a surface force originating from the thermal Marangoni effect. This actuator has been characterized (B. Selva, I. Cantat, and M.-C. Jullien, Migration of a bubble towards a higher surface tension under the effect of thermocapillary stress, preprint, 2009) and it was found that the bubble/droplet (called further element) is driven toward a high surface tension region, i.e. toward cold region, and the element velocity increases while decreasing the cavity thickness. Taking advantage of these properties three applications are presented: (1) element displacement, (2) element switching, detailed in a given range of working, in which elements are redirected towards a specific evacuation, (3) a system able to trap, and consequently stop on demand, the elements on an alveolus structure while the continuous phase is still flowing. The strength of this method lies in its simplicity: single layer system, in situ heating leading to a high level of integration, low power consumption (P < 0.4 W), low applied voltage (about 10 V), and finally this system is able to manipulate elements within a flow velocity up to 1 cm s(-1).

Hollow microneedle arrays for intradermal drug delivery and DNA electroporation.

The association of microneedles with electric pulses causing electroporation could result in an efficient and less painful delivery of drugs and DNA into the skin. Hollow conductive microneedles were used for (1) needle-free intradermal injection and (2) electric pulse application in order to achieve electric field in the superficial layers of the skin sufficient for electroporation. Microneedle array was used in combination with a vibratory inserter to disrupt the stratum corneum, thus piercing the skin. Effective injection of proteins into the skin was achieved, resulting in an immune response directed to the model antigen ovalbumin. However, when used both as microneedles to inject and as electrodes to apply the electric pulses, the setup showed several limitations for DNA electrotransfer. This could be due to the distribution of the electric field in the skin as shown by numerical calculations and/or the low dose of DNA injected. Further investigation of these parameters is needed in order to optimize minimally invasive DNA electrotransfer in the skin.

A versatile technology for droplet-based microfluidics: thermomechanical actuation.

We report on a versatile technique for microfluidic droplet manipulation that proves effective at every step: from droplet generation to propulsion to sorting, rearrangement or break-up. Non-wetting droplets are thermomechanically actuated in a microfluidic chip using local heating resistors. Controlled temperature variation induces local dilation of the PDMS wall above the resistor, which drives the droplet away from the hot (i.e. constricted) region (B. Selva, I. Cantat and M.-C. Jullien, Phys. Fluids, 2011, 23, 052002). Adapted placing and actuation of such resistors thus allow us to push forward, stop, store and release, or even break up droplets, at the price of low electric power consumption (<150 mW). We believe this technically accessible method to provide a useful tool for droplet microfluidics.

A Review of Heating and Temperature Control in Microfluidic Systems: Techniques and Applications.

This review presents an overview of the different techniques developed over the last decade to regulate the temperature within microfluidic systems. A variety of different approaches has been adopted, from external heating sources to Joule heating, microwaves or the use of lasers to cite just a few examples. The scope of the technical solutions developed to date is impressive and encompasses for instance temperature ramp rates ranging from 0.1 to 2,000 °C/s leading to homogeneous temperatures from -3 °C to 120 °C, and constant gradients from 6 to 40 °C/mm with a fair degree of accuracy. We also examine some recent strategies developed for applications such as digital microfluidics, where integration of a heating source to generate a temperature gradient offers control of a key parameter, without necessarily requiring great accuracy. Conversely, Temperature Gradient Focusing requires high accuracy in order to control both the concentration and separation of charged species. In addition, the Polymerase Chain Reaction requires both accuracy (homogeneous temperature) and integration to carry out demanding heating cycles. The spectrum of applications requiring temperature regulation is growing rapidly with increasingly important implications for the physical, chemical and biotechnological sectors, depending on the relevant heating technique.

A novel microfluidic flow-injection analysis device with fluorescence detection for cation sensing. Application to potassium.

A microfabricated device has been developed for fluorimetric detection of potassium ions without previous separation. It is based on use of a fluorescent molecular sensor, calix-bodipy, specially designed to be sensitive to and selective for the target ion. The device is essentially made of a Y-shape microchannel moulded in PDMS fixed on a glass substrate. A passive mixer is used for mixing the reactant and the analyte. The optical detection arrangement uses two optical fibres, one for excitation by a light-emitting diode, the other for collection of the fluorescence. This system enabled the flow-injection analysis of the concentration of potassium ions in aqueous solutions with a detection limit of 0.5 mmol L(-1) and without interference with sodium ions. A calibration plot was constructed using potassium standard solutions in the range 0-16 mmol L(-1), and was used for the determination of the potassium content of a pharmaceutical pill. Figure Photography of the microfluidic channel showing the ridges in the PDMS substrate at the top of the channel.