Continuous Manipulation and Characterization of Colloidal Beads and Liposomes via Diffusiophoresis in Single- and Double-Junction Microchannels

We reveal a physical mechanism that enables the preconcentration, sorting, and characterization of charged polystyrene nanobeads and liposomes dispersed in a continuous flow within a straight micron-sized channel. Initially, a single Ψ-junction microfluidic chip is used to generate a steady-state salt concentration gradient in the direction perpendicular to the flow. As a result, fluorescent nanobeads dispersed in the electrolyte solutions accumulate into symmetric regions of the channel, appearing as two distinct symmetric stripes when the channel is observed from the top via epi-fluorescence microscopy. Depending on the electrolyte flow configuration and, thus, the direction of the salt concentration gradient field, the fluorescent stripes get closer to or apart from each other as the distance from the inlet increases. Our numerical and experimental analysis shows that although nanoparticle diffusiophoresis and hydrodynamic effects are involved in the accumulation process, diffusio-osmosis along the top and bottom channel walls plays a crucial role in the observed particles dynamics. In addition, we developed a proof-of-concept double Ψ-junction microfluidic device that exploits this accumulation mechanism for the size-based separation and size detection of nanobeads as well as for the measurement of zeta potential and charged lipid composition of liposomes under continuous flow settings. This device is also used to investigate the effect of fluid-like or gel-like states of the lipid membranes on the liposome diffusiophoretic response. The proposed strategy for solute-driven manipulation and characterization of colloids has great potential for microfluidic bioanalytical testing applications, including bioparticle preconcentration, sorting, sensing, and analysis.

Three-Dimensionally Complex Phase Behavior and Collective Phenomena in Mixtures of Acoustically Powered Chiral Microspinners

The process of dynamic self-organization of small building blocks is fundamental to the emergent function of living systems and is characteristic of their out-of-equilibrium homeostasis. The ability to control the interactions of synthetic particles in large groups could lead to the realization of analogous macroscopic robotic systems with microscopic complexity. Rotationally induced self-organization has been observed in biological systems and modeled theoretically, but studies of fast, autonomously moving synthetic rotors remain rare. Here, we report switchable, out-of-equilibrium hydrodynamic assembly and phase separation in suspensions of acoustically powered chiral microspinners. Semiquantitative modeling suggests that three-dimensionally (3D) complex spinners interact through viscous and weakly inertial (streaming) flows. The interactions between spinners were studied over a range of densities to construct a phase diagram, which included gaseous dimer pairing at low density, collective rotation and multiphase separation at intermediate densities, and ultimately jamming at high density. The 3D chirality of the spinners leads to self-organization in parallel planes, forming a three-dimensionally hierarchical system that goes beyond the 2D systems that have so far been modeled computationally. Dense mixtures of spinners and passive tracer particles also show active-passive phase separation. These observations are consistent with recent theoretical predictions of the hydrodynamic coupling between rotlets generated by autonomous spinners and provide an exciting experimental window to the study of colloidal active matter and microrobotic systems.

Enhanced Accumulation of Colloidal Particles in Microgrooved Channels via Diffusiophoresis and Steady-State Electrolyte Flows

The delivery of colloidal particles in dead-end microstructures is very challenging, since these geometries do not allow net flows of particle-laden fluids; meanwhile, diffusive transport is slow and inefficient. Recently, we introduced a novel particle manipulation strategy, based on diffusiophoresis, whereby the salt concentration gradient between parallel electrolyte streams in a microgrooved channel induces the rapid (i.e., within minutes) and reversible accumulation, retention, and removal of colloidal particles in the microgrooves. In this study, we investigated the effects of salt contrast and groove depth on the accumulation process in silicon microgrooves and determined the experimental conditions that lead to a particle concentration peak of more than four times the concentration in the channel bulk. Also, we achieved an average particle concentration in the grooves of more than twice the concentration in the flowing streams and almost 2 orders of magnitude larger than the average concentration in the grooves in the absence of a salt concentration gradient. Analytical sufficient and necessary conditions for particle accumulation are also derived. Finally, we successfully tested the accumulation process in polydimethylsiloxane microgrooved channels, as they are less expensive to fabricate than silicon microgrooved substrates. The controlled and enhanced accumulation of colloidal particles in dead-end structures by solute concentration gradients has potential applications in soft matter and living systems, such as drug delivery, synthetic biology, and on-chip diagnostics.

Two-dimensional micromodels for studying the convective dissolution of carbon dioxide in 2D water-saturated porous media

Convective dissolution is a perennial trapping mechanism of carbon dioxide in geological formations saturated with an aqueous phase. This process, which couples dissolution of supercritical CO₂, convection of the liquid containing the dissolved CO₂, and mixing of the latter within the liquid, has so far not been studied in two-dimensional porous media. In order to do so, two-dimensional (2D) porous micromodels (patterned Hele-Shaw cells) have been fabricated from UV-curable NOA63 glue. NOA63 is used instead of PDMS, which is permeable to CO₂ and does not allow for a controlled no flux boundary condition at the walls. The novel fabrication protocol proposed here, based on the bonding of a patterned photo-lithographed NOA63 layer on a flat NOA63 base, shows good reproducibility regardless of the patterns' typical size, and allows for easy filling of the cell despite the small value of the gap. A pressure chamber allows pressurizing the CO₂ and outside of the flow cell up to 10 bars. Experiments were performed in 11 different porous media geometries. As expected, a gravitational fingering instability is observed upon injection of gaseous carbon dioxide in the cell, resulting in the downwards migration of dissolved CO₂ plumes through the 2D porous structure. The initial wavelength of the fingers is larger in the presence of a hexagonal lattice of pillars. This effect can be correctly predicted from the theory for the gravitational instability in a Hele-Shaw cell devoid of pillars, provided that the permeability of the hexagonal porous medium is considered in the theory instead of that of the Hele-Shaw cell. Fluctuations around the theoretical prediction observed in the data are mostly attributed to a hitherto unknown weak locking of the wavelength on the distance between closest pillars.

Deviations from classical droplet evaporation theory

In this article, we show that significant deviations from the classical quasi-steady models of droplet evaporation can arise solely due to transient effects in the gas phase. The problem of fully transient evaporation of a single droplet in an infinite atmosphere is solved in a generalized, dimensionless framework with explicitly stated assumptions. The differences between the classical quasi-steady and fully transient models are quantified for a wide range of the 10-dimensional input domain and a robust predictive tool to rapidly quantify this difference is reported. In extreme cases, the classical quasi-steady model can overpredict the droplet lifetime by 80%. This overprediction increases when the energy required to bring the droplet into equilibrium with its environment becomes small compared with the energy required to cool the space around the droplet and therefore establish the quasi-steady temperature field. In the general case, it is shown that two transient regimes emerge when a droplet is suddenly immersed into an atmosphere. Initially, the droplet vaporizes faster than classical models predict since the surrounding gas takes time to cool and to saturate with vapour. Towards the end of its life, the droplet vaporizes slower than expected since the region of cold vapour established in the early stages of evaporation remains and insulates the droplet.

Reversible Trapping of Colloids in Microgrooved Channels via Diffusiophoresis under Steady-State Solute Gradients

The controlled transport of colloids in dead-end structures is a key capability that can enable a wide range of applications, such as biochemical analysis, drug delivery, and underground oil recovery. This Letter presents a new trapping mechanism that allows the fast (i.e., within a few minutes) and reversible accumulation of submicron particles within dead-end microgrooves by means of parallel streams with different salinity level. For the first time, particle focusing in dead-end structures is achieved under steady-state gradients. Confocal microscopy analysis and numerical investigations show that the particles are trapped at a flow recirculation region within the grooves due to a combination of diffusiophoresis transport and hydrodynamic effects. Counterintuitively, the particle velocity at the focusing point is not vanishing and, hence, the particles are continuously transported in and out of the focusing point. The accumulation process is also reversible and one can cyclically trap and release the colloids by controlling the salt concentration of the streams via a flow switching valve.

Model of Collective Fish Behavior with Hydrodynamic Interactions

Fish schooling is often modeled with self-propelled particles subject to phenomenological behavioral rules. Although fish are known to sense and exploit flow features, these models usually neglect hydrodynamics. Here, we propose a novel model that couples behavioral rules with far-field hydrodynamic interactions. We show that (1) a new "collective turning" phase emerges, (2) on average, individuals swim faster thanks to the fluid, and (3) the flow enhances behavioral noise. The results of this model suggest that hydrodynamic effects should be considered to fully understand the collective dynamics of fish.

Small acoustically forced symmetric bodies in viscous fluids

The total force exerted on a small rigid body by an acoustic field in a viscous fluid is addressed analytically in the limit where the typical size of the particle is smaller than both the viscous diffusion length scale and the acoustic wavelength. In this low-frequency limit, such a force can be calculated provided the effect of the acoustic steady streaming is negligible. Using the Eulerian linear expansion of Lagrangian hydrodynamic quantities (velocity and pressure), the force on a small solid sphere free to move in an acoustic field is first calculated in the case of progressive and standing waves, and it is compared to past results. The proposed method is then extended to the case of more complex shapes with three planes of symmetry. For a symmetric body oriented with one of its axis along the wave direction, the acoustic force exerted by a progressive wave is affected by the particle shape at leading order. In contrast, for a standing wave (with the same orientation), the force experienced by the particle at leading order is the same as the one experienced by a sphere of same volume and density.

Does shaking increase the pressure inside a bottle of champagne?

Colas, beers and sparkling wines are all concentrated solutions of carbon dioxide in aqueous solvents. Any such carbonated liquid is ordinarily conditioned inside a closed bottle or a metal can as a liquid-gas 2-phase system. At thermodynamic equilibrium, the partial pressure of carbon-dioxide in the gas phase and its concentration in the liquid are proportional (Henry's law). In practical conditions and use (transport, opening of the container, exterior temperature change, etc.), Henry's equilibrium can be perturbed. The goal of this paper is to describe and understand how the system responds to such perturbations and evolves towards a new equilibrium state. Formally, we investigate the dynamics around Henry's equilibrium of a closed system, through dedicated experiments and modeling. We focus on the response to a sudden pressure change and to mechanical shaking (the latter point inspired the article's title). Observations are rationalized through basic considerations including molecular diffusion, bubble dynamics (based on Epstein-Plesset theory) and chemi-convective hydrodynamic instabilities.

Rotational propulsion enabled by inertia

The fluid mechanics of small-scale locomotion has recently attracted considerable attention, due to its importance in cell motility and the design of artificial micro-swimmers for biomedical applications. Most studies on the topic consider the ideal limit of zero Reynolds number. In this paper, we investigate a simple propulsion mechanism --an up-down asymmetric dumbbell rotating about its axis of symmetry-- unable to propel in the absence of inertia in a Newtonian fluid. Inertial forces lead to continuous propulsion for all finite values of the Reynolds number. We study computationally its propulsive characteristics as well as analytically in the small-Reynolds-number limit. We also derive the optimal dumbbell geometry. The direction of propulsion enabled by inertia is opposite to that induced by viscoelasticity.

Electrically induced microflows probed by fluorescence correlation spectroscopy

We report on the experimental characterisation of electrically induced flows at the micrometer scale through Fluorescence Correlation Spectroscopy (FCS) measurements. We stress the potential of FCS as a useful characterisation technique in microfluidics devices for transport properties cartography. The experimental results obtained in a model situation are in agreement with previous calculations (F. Nadal, F. Argoul, P. Kestener, B. Pouligny, C. Ybert, A. Ajdari, Eur. Phys. J. E 9, 387 (2002)) predicting the structure and electric-field dependency of the induced flow. Additionally, the present study evidences a complex behaviour of the probe nanobeads under electric field whose precise understanding might prove relevant for situations where nano-objects interact with an external electric field.

Electrically induced flows in the vicinity of a dielectric stripe on a conducting plane

We report a theoretical and experimental study of the hydrodynamic flow induced by an a.c. electric field in the vicinity of a dielectric stripe deposited on a conducting plate. In the theoretical part, we model the stripe as a small change of the surface capacitance of the plate, and a perturbative approach is used to perform the calculations. This approach predicts an outwards rectified electro-osmotic slip along the surface that generates two steady counter-rotating rolls, the size of which decreases with the frequency. In the experimental section, we use tracers to determine the structure of the flow and investigate its dependence on the frequency and the amplitude of the applied voltage. The structure and amplitude of the observed flow compares satisfactorily with the theoretical analysis. This could guide the design of surface-controlled flows and help to understand the collective behavior of colloids near electrodes.

Electrically induced interactions between colloidal particles in the vicinity of a conducting plane

We address the problem of two-dimensional (2D) colloidal aggregation driven by an ac electrical field, by observing an aqueous dispersion of latex microspheres in contact with a conducting surface. Using micron-sized carboxylated polystyrene particles, we have systematically investigated the aggregation process, as a function of particle size and charge, and of the applied electric field amplitude and frequency. A low-density 2D phase is observed at high frequency (typically above 1 kHz), while at low frequency (below a "contact frequency" nu(c)) the collection of particles collapses into disconnected compact aggregates of crystalline (hexagonal) structure. We argue that this scenario is governed by the competition between an attractive force, of electrohydrodynamic nature, and a repulsive force, basically an electrical dipole-dipole interaction. Both contributions are revealed and analyzed in independent experiments on isolated particle pairs, using optical manipulation and dynamometry.

Negative regulation of mitogen-activated protein kinase activation by integrin alphaIIbbeta3 in platelets

Activation of the mitogen-activated protein (MAP) kinase pathway in nucleated cells is dependent on both growth factor receptors and integrins engaged in cell adhesion. Human platelets are an interesting model for studying cell adhesion and the involvement of integrin engagement on extracellular signal-regulated kinase (ERK) activation, independently from the nuclear-DNA signal pathway. Maximal phosphorylation and activity of ERK2 occurred late during thrombin-induced platelet aggregation (90 s and later), an alphaIIbbeta3 integrin-dependent event. Surprisingly, alphaIIbbeta3 inhibition by the RGDS ligand peptide, or (Fab')2 fragments of the AP-2 monoclonal antibody, resulted in a 2-fold enhancement in ERK2 phosphorylation and activity. A similar 2-fold enhancement of ERK2 activation was observed in thrombasthenic platelets which are defective in alphaIIbbeta3 and do not aggregate. This suggests that ERK2 activation in thrombin-induced platelet aggregation is dependent on thrombin rather than on alphaIIbbeta3 and is down-regulated by alphaIIbbeta3 engaged in ligand (fibrinogen) binding and/or aggregation. Finally, in the absence of stirring which allows fibrinogen binding to alphaIIbbeta3 but prevents aggregation, ERK2 was again overactivated. This overactivation appears to be consecutive to inhibition of aggregation itself and to alphaIIbbeta3 ligand binding. We conclude that in platelets, alphaIIbbeta3 engaged in aggregation down-regulates thrombin-induced ERK2 activation. To our knowledge, this is the first report of a down-regulation of the MAP kinase pathway by integrin engagement.

[Hysteroscopic endometrial reduction. Analysis of 85 cases]

The authors report the results of their experience of hysteroscopic endometrial reduction through a retrospective series of 85 patients who underwent surgery between October 1988 and February 1991. The indication consisted of hemorrhage (menorrhagia and metrorrhagia) of endometrial origin in women with a group mean age of 46 years who had no intention of becoming pregnant in the future. The pre-treatment assessment was based on diagnostic hysteroscopy and was intended to eliminate neoplasm of the endometrium or its precursors. The endometrial reduction procedure always preserved an isthmic ring of endometrial tissue. No peroperative complication was reported. The mean duration of surgery was about 39 minutes. With a follow-up period of more than one year, bleeding had disappeared in 78% of cases. The authors conclude that the technic of endometrial resection in the treatment of benign lesions of the endometrium, on condition that the possibility of any disorder calling for laparotomy is ruled out and that there is regular postoperative follow-up.

[Triplet pregnancies in France. Results of a retrospective, multicenter study of two years (1987-1988). Proposals for optimal management]

The explosion of procedures for medically assisted parenthood (MAP) has resulted in a previously little known type of pregnancy: triple pregnancies. In order to assess the current obstetrical and pediatric situation, the authors have carried out a major retrospective, multicenter survey in France concerning the triple pregnancies from 1987 to 1988: 156 case histories have been collected. The finds are compared with those reported in the French and international literature, showing that in France, three-quarters of such pregnancies result from MAP, mainly due to ovulation-inducing agents. Hospitalization is prolonged (averaging 27 days), early (24 WA) and imposed by complications (in 8 out of 10 cases), prophylactic hospitalization being rarely prescribed. The main complications encountered are late miscarriages (1.9%), in-utero death (6.41%), dysgravidia (16.6%), hydramnios (5.12%) and serious cardio-pulmonary complications related to the use of beta-mimetics. A Cesarian is performed in only 87 percent of cases. The mean birth weight was 1,776 g. Neonatal mortality is on the decline, but still equivalent to 80.3 per thousand. Hypotrophy is common (27%). Birth is nearly always premature (99.3%), but very early prematurity (28-32 WA) was reduced. The approach suggested for optimum management is based on the personal experience of the authors, the findings of their survey and of the international literature. It is based on the prevention of prematurity and a fundamentally multidisciplinary approach.

[Indications for operative hysteroscopy. A series of 418 interventions]

Over a period of 38 months, from October 1, 1987 to December 31, 1990, 418 surgical hysteroscopies were carried out following systematic pre-operative hysteroscopy. Two-hundred and eighty-two patients presenting with a benign intercavitary lesion were treated by transcervical endo-uterine resection (EUR). The efficacy of the surgical method was assessed from the control of hemorrhagic phenomena, the most frequent sign. 89.7 percent of the patients who presented with menometrorrhagia, had become asymptomatic from the first cycle after EUR and 83.5 percent remained asymptomatic after a mean follow-up period of 28 months, 71 endometrectomies were carried out by EUR during this period, after careful hysteroscopic and histological evaluation of the endometrium. The results after one year of cycles were satisfactory in 82 percent of patients. Twenty-two patients presenting with a uterine septum were treated by endoscopic hysteroplasty. In most cases, the authors used the tip of the Charriere 21 resector. The follow-up period exceeds 1 year in 15 patients, 11 of whom became pregnant, giving birth to 9 live infants, 43 cases of synechia were treated by hysteroscopy in the context of Ashermann's syndrome. The functional results were good in 90 percent of patients, with the restoration of normal cycles. Only limited results were obtained in fertility in cases of muscular or fibrous synechia. The risks linked to surgical hysteroscopy are analysed. They appear to be acceptable, on condition that rigorous methods are used. The quality of sequelae and the short hospitalization make operative hysteroscopy preferable in the selected indications.

[Conservative treatment of cancer of the ovary]

The authors assess the potential of conservative treatment in a series of cases of ovarian cancer. Surgical evaluation is essential, in order to confirm that the conditions required for conservative treatment are present. The tumor must be a stage IA tumor, with a low histoprognostic score and small in size. Conservative treatment must be proposed in cases of tumors of potential malignancy and tumors of the sexual cords. It is debateable in epithelial tumors. The question of the necessity of further surgery to remove the genital tract after the desired pregnancies may be debateable in cases of epithelial tumors.

[Treatment of urinary stress incontinence by a modified Pereyra procedure:Chrub's operation]

The authors report the utility of suspending the cervix by means of the pubo-urethrax ligaments anchored to Cooper's ligaments in treating exercise-related urinary incontinence. A prospective series of 40 female patients gave 97.5% of successful outcomes with regard to the incontinence. The use of the pubo-urethrax ligaments and the absence of subsphincterian dissection confers major urodynamic advantages, particularly with regard to closing pressure. These anatomical and physiological arguments appear to give the procedure an advantage over suspensions using the vagina wall, whether these make use of the upper or lower route. Anchoring to Cooper's ligaments should ensure long-term stability of the outcome.