Synthesis of size-monodisperse spherical Ag@SiO2 nanoparticles and 3-D assembly assisted by microfluidics.

This article reports a one-step approach for the fabrication of highly uniform, spherical Ag particles with tailored dimensions ranging from 10 to 30 nm. Coated with silica shell, the high uniformity of the particles allows their spontaneous assembly into millimeter-long extended 3-D arrays with transverse dimensions of tens of micrometers, using a microfluidic evaporation-based process.

Solutal convection in confined geometries: enhancement of colloidal transport.

We evidence experimentally and theoretically that natural convection driven by solutal density differences in a molecular binary mixture can boost the transport of colloids. We demonstrate that such buoyancy-driven flows have a negligible influence on the gradients that generate them, for moderate Rayleigh numbers in a confined geometry. These flows therefore do not homogenize the binary mixture but can disperse very efficiently large solutes. We illustrate the relevance of such effects thanks to several original experiments: drying of confined droplets, microfluidic evaporation, and interdiffusion in microfluidic flows.

Evaporation of solutions and colloidal dispersions in confined droplets.

We present a model that describes the drying of solutions and colloidal dispersions from droplets confined between two circular plates. This confined geometry, proposed by Clément and Leng [Langmuir 20, 6538 (2004)], casts a perfect control of the evaporation conditions, and thus also of the concentration kinetics of the solutes in the droplet. Our model, based on simple transport equations for binary mixtures, describes the concentration process of the solute inside the droplet. Using dimensionless units, we identify the different numbers that govern the concentration field of the solute, and we detail how to extract kinetic and thermodynamic information on the binary mixture from such drying experiments. We finally discuss, using numerical resolution of the model and analytical arguments, several specific cases: dilute solutions, a colloidal hard sphere dispersion, and a binary molecular mixture.

Time-resolved microfocused small-angle X-ray scattering investigation of the microfluidic concentration of charged nanoparticles.

We describe the concentration process of a dispersion of silica nanoparticles undergoing evaporation in a dedicated microfluidic device. Using microfocused small-angle X-ray scattering, we measure in time and space both the concentration field of the dispersion and its structure factor. We show that the electrostatic interactions affect the concentration rate by strongly enhancing the collective diffusion coefficient of the nanoparticle dispersion. En route towards high concentrations, the nanoparticles eventually undergo a liquid-solid phase transition in which we evidence crystallites of micron size.

A spatio-temporal study of rheo-oscillations in a sheared lamellar phase using ultrasound.

We present an experimental study of the flow dynamics of a lamellar phase sheared in the Couette geometry. High-frequency ultrasonic pulses at 36 MHz are used to measure time-resolved velocity profiles. Oscillations of the viscosity occur in the vicinity of a shear-induced transition between a high-viscosity disordered fluid and a low-viscosity ordered fluid. The phase coexistence shows up as shear bands on the velocity profiles. We show that the dynamics of the rheological data result from two different processes: (i) fluctuations of slip velocities at the two walls and (ii) flow dynamics in the bulk of the lamellar phase. The bulk dynamics are shown to be related to the displacement of the interface between the two differently sheared regions in the gap of the Couette cell. Two different dynamical regimes are investigated under applied shear stress: one of small amplitude oscillations of the viscosity delta eta/eta approximately equal to 3%) and one of large oscillations (delta eta/eta approximately equal to 25%). A phenomenological model is proposed that may account for the observed spatio-temporal dynamics.

Towards local rheology of emulsions under Couette flow using Dynamic Light Scattering.

We present local velocity measurements in emulsions under shear using heterodyne Dynamic Light Scattering. Two emulsions are studied: a dilute system of volume fraction phi = 20% and a concentrated system with phi = 75%. Velocity profiles in both systems clearly show the presence of wall slip. We investigate the evolution of slip velocities as a function of shear stress and discuss the validity of the corrections for wall slip classically used in rheology. Focussing on the bulk flow, we show that the dilute system is Newtonian and that the concentrated emulsion is shear-thinning. In the latter case, the curvature of the velocity profiles is compatible with a shear-thinning exponent of 0.4 consistent with global rheological data. However, even if individual profiles can be accounted for by a power law fluid (with or without a yield stress), we could not find a fixed set of parameters that would fit the whole range of applied shear rates. Our data, thus, raise the question of the definition of a global flow curve for such a concentrated system. These results show that local measurements are a crucial complement to standard rheological tools. They are discussed in the light of recent works on soft glassy materials.

Comparison of commonly used clinical indicators of hypovolaemia with gastrointestinal tonometry.

OBJECTIVE: The gastrointestinal tonometer, which allows measurement of gastrointestinal mucosal CO2 and subsequent derivation of gut intramucosal pH (pHi), has been demonstrated to be a sensitive predictor of outcome following major surgery. Current theory suggests that the origin of the low pH may be hypovolaemia. This study was designed to compare the temporal sequence of changes in tonometric readings with invasive blood pressure, stroke volume, heart rate, lactate and arterial blood gas measurements during progressive haemorrhage. DESIGN: Observational healthy volunteer study. SETTING: Intensive care unit at University College London Hospitals. SUBJECTS: Six healthy, medically qualified volunteers. INTERVENTIONS: After obtaining baseline measurements, the subjects were progressively bled 25% (range = 21-31%) of their blood volume over a period of 1 h in two approximately equal aliquots. Equilibration was allowed for 30 min following the bleed, after which further measurements were made and the blood was then retransfused over 30 min. MEASUREMENTS AND MAIN RESULTS: There was no consistent change in any of the haemodynamic variables other than gastric intramucosal CO2:arterial CO2 gap (PiCO2-PaCO2) after removal of the first aliquot of blood, although five of the six subjects also demonstrated a fall in pHi. After removal of the second aliquot of blood, PiCO2-PaCO2 gap and pHi continued to indicate a worsening gastric intramucosal acidosis; stroke volume, as measured by suprasternal Doppler, demonstrated a marked fall, while all other variables measured had not altered consistently or to such a degree as to elicit a clinical response or cause suspicion of a hypovolaemic state. On retransfusion, all variables returned towards baseline. CONCLUSIONS: This study demonstrates the value of tonometry as an early monitor of hypovolaemia and highlights the shortcomings of other more commonly measured clinical variables.

The rational administration of colloids.

The safe administration of i.v. fluids is one of the most significant advances in the care of critically ill patients this century. However, despite advances in the monitoring of cardiovascular variables, the questions of what? when? and how much? remain areas of enormous controversy. Ironically as the choice of i.v. fluids becomes greater and the monitoring more sophisticated the controversy grows. This article will concentrate on the diagnosis of hypovolaemia, the consequences of hypovolaemia and the rational use of i.v. fluids, particularly colloids, to treat it.

Pharmacology and physiology of colloids.

The importance of an adequate circulating volume in the critically ill is well established. Plasma, albumin, synthetic colloids and crystalloids may all be used for volume expansion but the first two are expensive and crystalloids have to be given in much larger volumes than colloids to achieve the same effect. Synthetic colloids provide a cheaper, safe, effective alternative. There are three classes of synthetic colloid; dextrans, gelatins and hydroxyethyl starches; each is available in several formulations with different properties which affect their initial plasma expanding effects, retention in the circulation and side-effects. There is no ideal colloid but those with low molecular weights such as gelatins are more suitable for rapid, short term volume expansion whilst in states of capillary leak where longer term effects are required hydroxyethyl starches are more effective. Dextrans are as effective as the alternatives but produce more side-effects and the need to pre-treat with hapten-dextran renders them unwieldy in use. Albumin is as persistent as hydroxyethyl starch in the healthy circulation but is retained less well in states of capillary leak. It has no significant advantages over starches and is much more expensive.

Mapping the spread of epidural phenol in cancer pain patients by radionuclide admixture and epidural scintigraphy.

This study used a radionuclide imaging technique to map the spread and density contours of phenol in glycerin injected into the epidural space of cancer patients. Correlations were made between phenol injectate volume, sequence of injection, position of patient, and resultant epidural spread and analgesic outcome. Fifteen patients with cancer pain (average age of 61 years) were treated with serial epidural phenol in glycerin injections. Phenol in glycerin is miscible with [99mTc]sulfur colloid. An assumption was made that the admixture injected epidurally was inseparable prior to absorption and the spread was recorded by continuous gamma camera observations with computer collection for 30 min postinjection. The spread of injectate volumes of 2, 3, and 4 ml were compared and further correlations made between observed spread and sequence of injections (first to fifth in series within each patient) and position of patient. Small volumes of phenol may spread extensively in the epidural space (3 ml spreads a mean 13.6 segments) with wide variation among patients. Initial phenol injections spread further than subsequent injections. Maximum spread is achieved by 15 min postinjection and epidural distribution is mostly uniform, independent of patient position. Good analgesia was obtained in 14 patients (93%). Epidural neurolysis using serial injections of small volumes of phenol in glycerin is an effective, safe technique for cancer pain relief. Injectate volumes larger than 3 ml may be unnecessary and potentially dangerous.

Thalamic pain--the effect of electroconvulsive therapy (ECT).

An open study of the effect of a standard course of unilateral ECT applied to 4 patients with intractable thalamic pain. There was no significant change in pain, personality or affective profiles after treatment. Venous plasma endorphins were measured during the ECT course and there were no significant correlations with treatment.

Crystallization and subunit structure of histidine decarboxylase from Lactobacillus 30a.

Histidine decarboxylase from Lactobacillus 30a has been crystallized in a variety of forms which together indicate a revised subunit structure for the native particle. Octahedral crystals of the wild type enzyme obtained at room temperature from ammonium sulfate solutions in microdiffusion cells belong to tetragonal space group I4122 with a = b = 222 A and c = 107.5 A. Trigonal and hexagonal plates of prohistidine decarboxylase and activated proenzyme obtained at 4 degrees C from polyethyleneglycol solutions by vapor equilibration using the hanging drop technique belong to the trigonal space group P321 with a = b = 100 A and c = 164 A. The space group symmetries and unit cell contents of these crystals indicate 32 point group symmetry for the subunit structure of these enzymes. Sedimentation coefficients of wild type enzyme measured as a function of ionic strength at pH 7.0 indicate a rapid equilibrium between species varying from 6.9 S to 9.4 S. Sedimentation equilibrium analysis demonstrated the existence of a nearly homogeneous particle with Mr congruent to 208,000 at ionic strengths above I = 0.20, while an additional species of approximately one-half that molecular weight is observed at very ionic strengths (I = 0.2). At the pH optimum of the enzyme (pH 4.8), te larger species is dominant at all ionic strengths tested. Electron micrographs of native wild type enzyme show a dominant tetrahedral particle approximately 60 A on an edge while similar micrographs of enzyme cross-linked with glutaraldehyde show a dumbbell-shaped particle approximately 60 A in width and 120 A in length. These results establish that: (a) the native enzyme has a Mr congruent to 208,000 and a subunit composition (alpha beta)6; (b) the proenzyme has a subunit composition (pi)6; and (c) stable (alpha beta)3 and (pi) 3 particles exist under certain conditions.