Microevaporators for Kinetic Exploration of Phase Diagrams

Abstract

We use pervaporation-based microfluidic devices to concentrate species in aqueous solutions with spatial and temporal control of the process. Using experiments and modelling, we quantitatively describe the advection-diffusion behavior of the concentration field of various solutions (electrolytes, colloids, etc) and demonstrate the potential of these devices as universal tools for the kinetic exploration of the phases and textures that form upon concentration.PACS numbers: 07.90.+c, 64.75.+g Determination of the phase diagram of multicomponent systems is of importance in many realms: industrial formulation, protein cristallization, bottom up material assembly from spontaneous ordering of surfactant, polymeric or colloidal systems [1,2,3]. Depending on the application, one may want to access only the equilibrium phase diagram or gain additional information as to the metastable phases that can appear for kinetic reasons. Methods to reach these goals often imply tedious and systematic measurements, requiring for screening purposes the use of robotic platforms. Two generic strategies consist in varying (in space or time) the temperature of samples of given concentrations on the one hand, and on the other hand isothermal concentration by either removal of the solvent (osmosis, drying), external action on the solutes (sedimentation or dielectrophoresis for colloids), or studies of spontaneous interdiffusion in contact experiments.In this Letter we introduce microfluidic tools for controlled isothermal concentration of a wide range of systems, covering solutions of ions, polymers, proteins, surfactants and colloidal suspensions. Our work is inspired by recent observations [4,5] that in standard microsystems built of PolyDiMethylSiloxane (PDMS), spontaneous water permeation through the PDMS matrix induces flows that can be used to concentrate colloids. Taking a step further, we have engineered specialized microgeometries that allow us to control spatially and temporally the evaporation process as well as the resulting concentration of solutes. Their parallel implementation in microfluidic format could open the way to fast screening methods.After a brief description of the micro-devices, we demonstrate first our control of the concentration process on dilute aqueous solutions of fluorescein and nanoparticles in a simple geometry. We then discuss how microfabrication permits to widen the range of possibili- ties and applications of such devices. As a study case, we report controlled nucleation and growth of crystals of potassium chloride (KCl), and show how such experiments provide quantitative information on various thermodynamic quantities (solubility, crystal density) as well as kinetic features (sensitive to the rate of concentration).The devices -The devices used in this paper are twolayer PDMS on glass microsystems (Fig. 1) (fabrication procedure detailed in [6]). The microchannels of the bottom layer are filled with the solution of interest, while air (at controlled humidity) is circulated through the microchan...

Rapid evaporation-driven chemical pre-concentration and separation on paper

“…Following the observation of permeation-induced flows in poly(dimethyl siloxane) [31,32], airflow-enhanced PDMS pervaporation pumps were used to induce concentration in microfluidic channels [33][34][35].…”

Microfluidic Exploration of the Phase Diagram of a Surfactant/Water Binary System

“…microfluidic chips) is an attractive avenue to increase productivity. We have recently introduced microevaporators (devices with integrated pervaporation) for the concentration of small volumes of aqueous solutions [4], and demonstrated the great level of control brought up by miniaturization on very dilute solutions, and in a study of the apparition and growth of crystals in a simple electrolyte (KCl) solution.…”

“…Growth kinetics -We now show how analysis of the kinetics permits the quantitative determination of a phase boundary for this complex systems, applying a recipe succesful for electrolyte solutions [4].…”

“…microfluidic chips) is an attractive avenue to increase productivity. We have recently introduced microevaporators (devices with integrated pervaporation) for the concentration of small volumes of aqueous solutions [4], and demonstrated the great level of control brought up by miniaturization on very dilute solutions, and in a study of the apparition and growth of crystals in a simple electrolyte (KCl) solution.In this Letter we go one step further, and show that these devices permit the screening of the properties of more complex systems, here a binary surfactant/water mixture that exhibits several phase transitions en route towards high concentrations. We indeed report below the successive occurence of the four phases expected at equilibrium, and also out-of-equilibrium features related to the densification process.…”

“…A set of 48 dead-end microchannels (thickness h ∼ 25µm), originating from a common reservoir, run over the membrane (a thin PDMS layer of thickness e ∼ 10 µm), below which a wide channel allows removal of the water that permeates through this membrane. The channels have a gradually varying length L 0 overlying the membrane, which yields a gradient of concentration rate [4].Indeed, if water permeates at a volumetric flow rate v e through the bottom membrane, the compensating flow from the reservoir has an average velocity v 0 = v e L 0 /h. If the reservoir is filled with a dilute solution at concentration c 0 the flux of solute in the microchannel is j 0 = c 0 v 0 and thus increases with L 0 .…”