Florent Malloggi

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.

Monodisperse Colloids Synthesized with Nanofluidic Technology

Limitations in the methods employed to generate micrometric colloidal droplets hinder the emergence of key applications in the fields of material science and drug delivery. Through the use of dedicated nanofluidic devices and by taking advantage of an original physical effect called capillary focusing, we could circumvent some of these limitations. The nanofluidic (i.e., submicrometric) devices introduced herein are made of soft materials, and their fabrication relies upon rapid technologies. The objects that we have generated are simple droplets, multiple droplets, particles, and Janus particles whose sizes lie between 900 nm and 3 microm (i.e., within the colloidal range). Colloidal droplets have been assembled on-chip into clusters and crystals, yielding discrete diffraction patterns. We illustrate potential applications in the field of drug delivery by demonstrating the ability of multiple droplets to be phagocytosed by murine macrophage-type cells.

Cusps, spouts and microfiber synthesis with microfluidics

We produced jets of two immiscible liquids in a standard microfluidic flow focusing geometry, using semi-dilute aqueous polymer solutions as the external phase and immiscible liquids, oil or photocurable polymers as the internal one. We map out the different flow regimes on a “phase diagram”. Two new flow regimes – oscillatory jet and spout – are observed, as a result of the non-Newtonian behavior of the external phase. In the spout regime, cusp-shaped interfaces form at the junction, emitting extremely small round jets (spouts), with diameters ranging between 1.2 and 7 μm, i.e. an order of magnitude smaller than the microchannel cross-sectional dimensions. These spouts are found to be stable over remarkably long distances. We analyze the properties of the cusps and the spouts in some detail. The system can be utilized to synthesize fibers of micrometric sizes: the fibers we obtained, under different flow conditions, have diameters ranging between 1.8 and 14 μm and lengths ranging between 0.5 mm and 2 centimeters.

Electrolytes at interfaces: accessing the first nanometers using X-ray standing waves

Ion-surface interactions are of high practical importance in a wide range of technological, environmental and biological problems. In particular, they ultimately control the electric double layer structure, hence the interaction between particles in aqueous solutions. Despite numerous achievements, progress in their understanding is still limited by the lack of experimental determination of the surface composition with appropriate resolution. Tackling this challenge, we have developed a method based on X-ray standing waves coupled to nano-confinement which allows the determination of ion concentrations at a solid-solution interface with a sub-nm resolution. We have investigated mixtures of KCl/CsCl and KCl/KI in 0.1 mM to 10 mM concentrations on silica surfaces and obtained quantitative information on the partition of ions between bulk and Stern layer as well as their distribution in the Stern layer. Regarding partition of potassium ions, our results are in agreement with a recent AFM study. We show that in a mixture of KCl and KI, chloride ions exhibit a higher surface propensity than iodide ions, having a higher concentration within the Stern layer and being on average closer to the surface by ≈1-2 Å, in contrast to the solution water interface. Confronting such data with molecular simulations will lead to a precise understanding of ionic distributions at aqueous interfaces.

DMF-MALDI: droplet based microfluidic combined to MALDI-TOF for focused peptide detection

We present an automated droplet microfluidic system (DMF) to generate monitored nanoliter aqueous droplets in oil and their deposition on a commercial stainless steel plate for MALDI-TOF analysis of peptides or protein digests. We demonstrate that DMF-MALDI combination focuses the analyte on the MALDI plate, increasing considerably the homogeneity of the dried material. This results in a 30times enhanced MALDI-TOF MS signal for a model peptide, allowing a significant improvement of the detection sensitivity limit (down to few tens of attomoles). Moreover, positive detection can be achieved from sub-nanomolar peptides solutions and better overall protein sequence coverages are obtained from few tens attomoles of protein digest. These results make DMF-MALDI a promising approach for the treatment of peptides samples as well as a key component for an integrated approach in the proteomic field.

Marker patterning: a spatially resolved method for tuning the wettability of PDMS

Versatile, inexpensive and easy to use, PDMS has become a very common material, especially in the field of microfluidics. Intrinsically hydrophobic, it can be made hydrophilic by exposure to oxygen plasma. For some applications, wettability patterning of the PDMS surface is needed, in particular for the formation of monodisperse multiple emulsions. In this work we present an easy and spatially resolved way to tune the wettability of PDMS. Patterning is achieved without tedious or complicated surface processing by simply drawing the desired pattern onto the PDMS surface using the permanent ink of a marker as a masking layer. The microfluidic device is then exposed to oxygen plasma prior to bonding and flushed with ethanol when bonded. The parts of PDMS which are protected remained hydrophobic whereas unprotected surfaces are oxidized. The process is demonstrated by forming W/O/W emulsions in a controlled manner.