Microdroplet Approach for Measuring Aqueous Solubility and Nucleation Kinetics of a Metastable Polymorph: The case of KDP Phase IV

Solubility and interfacial energy are two fundamental parameters underlying the competitive nucleation of polymorphs. However, solubility measurement of metastable phases comes with a risk of solventmediated transformations which can render the results unreliable. In this work, we present a rapid microfluidic technique for measuring aqueous solubility of the metastable form using KDP Phase IV as a model system. This bracketing approach involves analyzing the dissolution behavior of crystals in contact with supersaturated microdroplets generated via evaporation. Then, with the help of our recently developed nucleation time measurement technique, together with Mersmann calculation of interfacial energies from solubilities, we were able to access the interfacial energies of both metastable and stable phases. To gain further insights into the observed nucleation behavior, we employed the Classical Nucleation Theory (CNT) to model the competition of polymorphs using our measured solubility and calculated interfacial energies. The results show that the stable form is favored at lower supersaturation while the metastable form is favored at higher supersaturation, in good agreement with our observations and experimental reports in the literature. Overall, our microfluidic approach allows access to unprecedentedly deep levels of supersaturation and reveals an interesting interplay between thermodynamics and kinetics in polymorphic nucleation. The experimental methods and insights presented herein can be of great interest, notably in the mineral processing and pharmaceutical industry

CNT effective interfacial energy and pre-exponential kinetic factor from measured NaCl crystal nucleation time distributions in contracting microdroplets

Nucleation, the birth of a stable cluster from disorder, is inherently stochastic. Yet up to date, there are no quantitative studies on NaCl nucleation that accounts for its stochastic nature. Here, we report the first stochastic treatment of NaCl-water nucleation kinetics. Using a recently developed microfluidic system and evaporation model, our measured interfacial energies extracted from a modified Poisson distribution of nucleation time show an excellent agreement with theoretical predictions. Furthermore, analysis of nucleation parameters in 0.5 pL, 1.5pL and 5.5 pL microdroplets reveals an interesting interplay between kinetic confinement and shifting of nucleation mechanisms. Overall, our findings highlight the need to treat nucleation stochastically rather deterministically to bridge the gap between theory and experiment

A microfluidic method generating monodispersed microparticles with controllable sizes and mechanical properties

International audienceSeeking to produce microparticles that mimic red blood cells (RBCs), we present a microfluidic method of generating monodispersed hydrogel microparticles of Na-/Ca-alginate with controllable sizes (micrometer range) and mechanical properties. No surfactant is used. Transformation from Na-alginate to Ca-alginate microparticles is realized via ex situ gelation, which proves essential to obtaining desired microparticle properties, such as insolubility in water and RBC-like mechanical properties. For both Na-alginate and Ca-alginate microparticles, a smooth surface and a porous inner structure are observed under a scanning electron microscope. A platform of microgrippers is successfully developed to manipulate the microparticles. The Young’s modulus measured using an atomic force microscope on the surface of Ca-alginate microparticles is of the same order as that of RBCs

Are conductance plateaus independent events in atomic point contact measurements ? A statistical approach

Room temperature conductance-elongation curves of gold atomic wires are measured using a Scanning Tunnelling Microscope Break Junction technique. Landauer's conductance plateaus are individually identified and statistically analysed. Both the probabilities to observe, and the lengths of the two last plateaus (at conductance values close to 2e2/h and 4e2/h) are studied. All results converge to show that the occurrence of these two conductance plateaus on a conductance-elongation curve are statistically independent events.Comment: 8 pages, 5 figures. Accepted for publication in Nanotechnolog

Influence of Dynamic Surface Tension on the Spreading of Surfactant Solution Droplets Impacting onto a Low-Surface-Energy Solid Substrate

International audienceWe have investigated the impact of single droplets of various surfactant solutions on a low-surface-energy solid substrate using a high-frequency visualization technique (one picture every 100 ms ) . Whatever the surfactant, the drop spreads and retracts in about 1 s under the action of inertia and capillarity, respectivelyDuring retraction, the capillary waves can be amplified and, in some cases, even yield droplet bouncing. Then, the droplet may slowly spread again due to gravity and the unbalanced capillary forces at the contact line between the droplet and the substrate. During the fast spreading process (2–3 ms), the droplet surface increases by almost one order of magnitude since its shape changes from a sphere to a flat pancake; this causes a strong deviation from thermodynamic equilibrium. The relevant surface property is therefore the dynamic surface tension which we have evaluated using a maximum bubble pressure apparatus. We have shown that droplet retraction is drastically influenced by the adsorption kinetics of the surfactant which limits the return to equilibrium surface tensio

Détection et Physico-chimie d'Objets Uniques

This HDR manuscript contains 3 chapters presenting my main research activities as Associate Professor in Centre Interdisciplinaire de Nanosciences de Marseille (CINaM): - Bio-fonctionalized surfaces for individual molecules grafting and detection - Atomic and molecular junctions - Nucleation of a mono-cristal in a microfluidic systemCe manuscrit d'HDR comporte 3 chapitres présentant mes principales activités de recherche depuis ma prise de fonction de maître de Conférence au Centre Interdisciplinaire de Nanosciences de Marseille (CINaM): - Surfaces bio-fonctionnalisées pour le greffage et la détection de molécules uniques - Contacts atomiques et moléculaires - Nucléation d'un cristal unique dans un système microfluidiqu

Advances in the Use of Microfluidics to Study Crystallization Fundamentals

International audienceThis review compares droplet-based microfluidic systems used to study crystallization fundamentals in chemistry and biology. An original high-throughput droplet-based microfluidic platform is presented. It uses nL droplets, generates a "chemical library" and directly solubilizes powder, thus economizing both material and time. It is compatible with all solvents without the need for surfactant. Its flexibility permits phase diagram determination and crystallization studies (screening and optimizing experiments), and makes it easy to use for non-specialists in microfluidics. Moreover, it allows concentration measurement via UV spectroscopy and solid characterization via XRD analysis

Preparation of alginate hydrogel microparticles by gelation introducing cross-linkers using droplet-based microfluidics: a review of methods

International audienceThis review examines the preparation of alginate hydrogel microparticles by using droplet-based microfluidics, a technique widely employed for its ease of use and excellent control of physicochemical properties, with narrow size distribution. The gelation of alginate is realized “on-chip” and/or “off-chip”, depending on where cross-linkers are introduced. Various strategies are described and compared. Microparticle properties such as size, shape, concentration, stability and mechanical properties are discussed. Finally, we consider future perspectives for the preparation of hydrogel microparticles and their potential applications