Fabrice Monti

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.

How do soft particle glasses yield and flow near solid surfaces

We use fluorescence microscopy and particle tracking velocimetry to image the motion of concentrated emulsions and microgel suspensions near solid surfaces. The local deformation involves a combination of slip and bulk flow, which are found to be controlled by surface forces. With smooth surfaces, two slip mechanisms are identified depending on whether particle–wall interactions are repulsive or weakly attractive. In the former case, the materials yield uniformly and the local rheology can be mapped on the macroscopic flow curve. In the latter case, yielding is non-uniform which reveals a continuous distribution of states from the immediate vicinity of the smooth surface to the bulk of the material. The effect of the surface is long-ranged and decays exponentially with the distance, which can be described by a non-local fluidity model. Our results establish a link between surface forces, lubrication and yielding in soft glassy or jammed materials and open new routes to manipulate their flow through the surface chemistry of the confining boundaries.

Doubly Responsive Polymer-Microgel Composites: Rheology and Structure

Mixtures of alkali swellable microgels and linear PNIPAm chains exhibit doubly responsive properties both with pH and temperature. Below the lower critical solution temperature (LCST), the linear chains of PNIPAm are soluble and increase the osmotic pressure outside the microgels, which causes them to deswell. Above the LCST, the PNIPAm chains become insoluble and form spherical colloidal particles confined between the microgels that subsequently reswell. The swelling and deswelling of the microgels change the rheological properties of the composites, providing a unique way to tune the elasticity of the composites with temperature. The structure of the composites above the LCST is studied using multiple light scattering and fluorescence confocal microscopy. The phase separation of PNIPAm above the LCST is strongly affected by the confinement of the PNIPAm chains between the microgels.

High spatiotemporal control of spontaneous reactions using ultrasound-triggered composite droplets.

Achieving high spatial and temporal control over a spontaneous reaction is a particularly challenging task with potential breakthroughs in various fields of research including surface patterning and drug delivery. We report here an exceptionally effective method that allows attaining such control. This method relies on a remotely triggered ultrasound-induced release of a reactant encapsulated in a composite microdroplet of liquid perfluorohexane. More specifically, the demonstration was achieved by locally applying a focused 2.25 MHz transducer onto a microfluidic channel in which were injected composite microdroplets containing a solution of an azidocoumarin and an external flow containing a reactive alkyne.

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.

Particle deposition kinetics of colloidal suspensions in microchannels at high ionic strength

Despite its considerable practical importance, the deposition of real Brownian particles transported in a channel by a liquid, at small Reynolds numbers, has never been described at a comprehensive level. Here, by coupling microfluidic experiments, theory, and numerics, we succeed in unravelling the problem for the case of straight channels at high salinity. We discover a broad regime of deposition (the van der Waals regime) in which particle-wall van der Waals interactions govern the deposition mechanism. We determine the range of existence of the regime, for which we calculate the concentration profiles, retention profiles, and deposition kinetics analytically. The retention profiles decay as the inverse of the square root of the distance from the entry, and the deposition kinetics are given by the expression [Formula: see text], where S is a dimensionless deposition function, A is the Hamaker constant, and ξL is a dimensionless parameter characterizing fluid flow properties. These findings are well supported by numerics. Experimentally, we find that the retention profiles behave as x-0.5±0.1 (where x is the distance from the channel entry) over three decades in scale, as predicted theoretically. By varying the flow conditions (speed, geometry, surface properties, and concentration) so as to cover four decades in ξL and taking the Hamaker constant as a free parameter, we accurately confirm the theoretical expression for the deposition kinetics. Operating in the van der Waals regime enables control of the deposition rates via surface chemistry. From a surface science perspective, working in the van der Waals regime enables us to measure the Hamaker constants of thousands of particles in a few minutes, a task that would take a much longer time to perform with standard AFM.

In situ targeted activation of an anticancer agent using ultrasound-triggered release of composite droplets

The efficiency of a drug is usually highly dependent on the way it is administered or delivered. As such, targeted-therapy, which requires conceiving drug-delivery vehicles that will change their state from a relatively stable structure with a very slow leak-rate to an unstable structure with a fast release, clearly improves the pharmacokinetics, the absorption, the distribution, the metabolism and the therapeutic index of a given drug. In this context, we have developed a particularly effective double stimuli-responsive drug-delivery method allowing an ultrasound-induced release of a monomethylauristatin E-glucuronide prodrug and its subsequent activation by a β-glucuronidase. This led to an increase of cytotoxicity of about 80% on cancer cells.

Thin-Film Heat-Flux Microsensor for Heat-Transfer Measurement in Micro-Heat Exchangers/Microreactors

Heat-transfer analysis in microfluidic devices is of great importance in applications such as micro-heat exchangers and microreactors. This work reports on improvements in temperature measurement techniques, which can be the source of large errors due to their intrusiveness and the unreliability of conventional thermal sensors. Gold thin films were deposited on a borosilicate substrate to realize a 2D heat flux sensor for heat-transfer measurement along the main flow within microchannels. Two applications are shown, one related to micro-heat exchangers and the other to microreactors. For the micro-heat exchanger, the effect of length scale on heat transfer in a straight microchannel was investigated and the validity of macroscale correlations for convective heat transfer was checked for deionized water flowing in microchannels of heights 12 to 52 μm. For the microreactor, the reaction enthalpy of an acid–base reaction measured using the new heat-flux sensor had only a 5% discrepancy from the standard value, showing the efficiency of the new thin-film device.Copyright