Marie-Caroline Jullien

Droplet breakup in microfluidic T-junctions at small capillary numbers

We perform experimental studies of droplet breakup in microfluidic T-junctions in a range of capillary numbers lying between 4×10−4 and 2×10−1 and for two viscosity ratios of the fluids forming the dispersed and continuous phases. The present paper extends the range of capillary numbers explored by previous investigators by two orders of magnitude. We single out two different regimes of breakup. In a first regime, a gap exists between the droplet and the wall before breakup occurs. In this case, the breakup process agrees well with the analytical theory of Leshansky and Pismen [Phys. Fluids 21, 023303 (2009)]. In a second regime, droplets keep obstructing the T-junction before breakup. Using physical arguments, we introduce a critical droplet extension for describing the breakup process in this case.

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.

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.

An optimized resistor pattern for temperature gradient control in microfluidics

In this paper, we demonstrate the possibility of generating high-temperature gradients with a linear temperature profile when heating is provided in situ. Thanks to improved optimization algorithms, the shape of resistors, which constitute the heating source, is optimized by applying the genetic algorithm NSGA-II (acronym for the non-dominated sorting genetic algorithm) (Deb et al 2002 IEEE Trans. Evol. Comput. 6 2). Experimental validation of the linear temperature profile within the cavity is carried out using a thermally sensitive fluorophore, called Rhodamine B (Ross et al 2001 Anal. Chem. 73 4117–23, Erickson et al 2003 Lab Chip 3 141–9). The high level of agreement obtained between experimental and numerical results serves to validate the accuracy of this method for generating highly controlled temperature profiles. In the field of actuation, such a device is of potential interest since it allows for controlling bubbles or droplets moving by means of thermocapillary effects (Baroud et al 2007 Phys. Rev. E 75 046302). Digital microfluidics is a critical area in the field of microfluidics (Dreyfus et al 2003 Phys. Rev. Lett. 90 14) as well as in the so-called lab-on-a-chip technology. Through an example, the large application potential of such a technique is demonstrated, which entails handling a single bubble driven along a cavity using simple and tunable embedded resistors.

Temperature-induced migration of a bubble in a soft microcavity

We perform studies of pancake-like shaped bubbles submitted to a temperature gradient in a micrometric height Hele-Shaw cell. We show that under the experimental conditions, usually found in microfluidic devices, the temperature-induced dilation of the cavity overcomes the thermocapillary convection due to surface tension variation, effectively driving the bubble toward the cold side of the cavity. The bubble velocity is experimentally characterized as a function of the bubble radius, the temperature gradient, and the initial Hele-Shaw cell thickness. We propose a theoretical prediction of the bubble velocity, based on the analytical resolution of the hydrodynamical problem. The equations set closure is ensured by the pressure value near the bubble and by the dissipation in the moving meniscus.

2D foam coarsening in a microfluidic system

We report an experimental study of 2D microfoam coarsening confined in a micrometer scale geometry, the typical bubbles diameter being of the order of 50–100 μm. These experiments raise both fundamental and applicative issues. For applicative issues: what is the typical time of foam ageing (for a polydisperse foam) in microsystems in scope of gas pocket storage in lab-on-a-chips? Experimental results show that a typical time of 2–3 mn is found, leading to the possibility of short-time storing, depending on the application. For fundamental interests, 2D foam ageing is generally described by von Neumanns law (von Neumann J., Metal Interfaces (American Society of Metals, Cleveland) 1952, p. 108) which is based on the hypothesis that bubbles are separated by thin films. Does this hypothesis still hold for foams confined in a 40 μm height geometry? This problematic is analyzed and it is shown that von Neumanns law still holds but that the diffusion coefficient involved in this law is modified by the confinement which imposes a curvature radius at Plateau borders. More precisely, it is shown that the liquid fraction is high on a film cross-section, in contrast with macrometric experiments where drainage occurs. An analytical description of the diffusion is developped taking into account the fact that soap film height is only a fraction of the cell height. While most of microfoams are flowing, the experimental set-up we describe leads to the achievement of a motionless confined microfoam.

A microfluidic distribution system for an array of hollow microneedles

We report a microfluidic device able to control the ejection of fluid through a matrix of out-of-plane microneedles. The device comprises a matrix of open dispensing units connected to needles and filled by a common filling system. A deformable membrane (e.g. in PDMS) is brought into contact with the dispensing units. Pressure exerted on the deformable membrane closes (and thus individualizes) each dispensing unit and provokes the ejection of the dispensing unit content through the outlets. Sufficient pressure over the deformable membrane ensures that all dispensing units deliver a fixed volume (their content) irrespective of the hydrodynamic pressure outside the dispensing unit outlet. The size of the ensemble matrix of dispensing units, the number of liquid reservoirs, as well as the material can vary depending on the considered application of the device or on the conditions of use. In the present paper, the liquid reservoirs are geometrically identical. The geometrical parameters of the device are optimized to avoid as much dead volume as possible, as it was to handle plasmid DNA solutions which are very expensive. The conception, the fabrication and the experimental results are described in this paper. Our prototype is conceived to inject in a uniform way 10 µl of drug through 100 microneedles distributed over 1 cm2.

Laplace pressure based disjoining pressure isotherm in non symmetric conditions

Understanding the stability and dynamics of two phase systems, such as foams and emulsions, in porous media is still a challenge for physicists and calls for a better understanding of the intermolecular interactions between interfaces. In a classical approach, these interactions are investigated in the framework of Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory by building disjoining pressure isotherms. This paper reports on a technique allowing the measurement of disjoining pressure isotherms in a thin liquid film squeezed by either a gas or a liquid phase on a solid substrate. We couple a Reflection Interference Contrast Microscopy set-up to a microfluidic channel that sets the disjoining pressure through the Laplace pressure. This simple technique is found to be both accurate and precise. The Laplace pressure mechanism provides extremely stable conditions and offers opportunity for parallelizing experiments by producing several drops in channels of different heights. We illustrate its potential ...