Pierre Joseph

Slippage of water past superhydrophobic carbon nanotube forests in microchannels.

We present in this Letter an experimental characterization of liquid flow slippage over superhydrophobic surfaces made of carbon nanotube forests, incorporated in microchannels. We make use of a particle image velocimetry technique to achieve the submicrometric resolution on the flow profile necessary for accurate measurement of the surface hydrodynamic properties. We demonstrate boundary slippage on the Cassie superhydrophobic state, associated with slip lengths of a few microns, while a vanishing slip length is found in the Wenzel state when the liquid impregnates the surface. Varying the lateral roughness scale L of our carbon nanotube forest-based superhydrophobic surfaces, we demonstrate that the slip length varies linearly with L in line with theoretical predictions for slippage on patterned surfaces.

Achieving large slip with superhydrophobic surfaces: Scaling laws for generic geometries

We investigate the hydrodynamic friction properties of superhydrophobic surfaces and quantify their superlubricating potential. On such surfaces, the contact of the liquid with the solid roughness is minimal, while most of the interface is a liquid-gas one, resulting in strongly reduced friction. We obtain scaling laws for the effective slip length at the surface in terms of the generic surface characteristics (roughness length scale, depth, solid fraction of the interface, etc.). These predictions are successfully compared to numerical results in various geometries (grooves, posts or holes). This approach provides a versatile framework for the description of slip on these composite surfaces. Slip lengths up to 100μm are predicted for an optimized patterned surface.

Second-order slip laws in microchannels for helium and nitrogen

We perform gas flow experiments in a shallow microchannel, 1.14±0.02 μm deep, 200 μm wide, etched in glass and covered by an atomically flat silicon wafer. The dimensions of the channel are accurately measured by using profilometry, optical microscopy and interferometric optical microscopy. Flow-rate and pressure drop measurements are performed for helium and nitrogen, in a range of averaged Knudsen numbers extending up to 0.8 for helium and 0.6 for nitrogen. This represents an extension, by a factor of 3 or so, of previous studies. We emphasize the importance of the averaged Knudsen number which is identified as the basic control parameter of the problem. From the measurements, we estimate the accommodation factor for helium to be equal to 0.91±0.03 and that for nitrogen equal to 0.87±0.06. We provide estimates for second-order effects, and compare them with theoretical expectations. We estimate the upper limit of the slip flow regime, in terms of the averaged Knudsen number, to be 0.3±0.1, for the two gases.

Respiratory variations in pulse oximetry plethysmographic waveform amplitude to predict fluid responsiveness in the operating room.

Background:Respiratory variations in pulse oximetry plethysmographic waveform amplitude (&Dgr;POP) are related to respiratory variations in pulse pressure (&Dgr;PP) and are sensitive to changes in preload. The authors hypothesized that &Dgr;POP can predict fluid responsiveness in mechanically ventilated patients during general anesthesia. Methods:Twenty-five patients referred for cardiac surgery were studied after induction of general anesthesia. Hemodynamic data (cardiac index, central venous pressure, pulmonary capillary wedge pressure, &Dgr;PP, and &Dgr;POP) were recorded before and after volume expansion (500 ml hetastarch, 6%). Fluid responsiveness was defined as an increase in cardiac index of 15% or greater. Results:Volume expansion induced changes in cardiac index (2.0 ± 0.4 to 2.3 ± 0.5 mmHg; P < 0.05), &Dgr;PP (11 ± 7 to 6 ± 5%; P < 0.05), and &Dgr;POP (12 ± 9 to 7 ± 5%; P < 0.05). &Dgr;POP and &Dgr;PP were higher in responders than in nonresponders (17 ± 8 vs. 6 ± 4 and 14 ± 7 vs. 6 ± 4%, respectively; P < 0.05 for both). A &Dgr;POP greater than 13% before volume expansion allowed discrimination between responders and nonresponders with 80% sensitivity and 90% specificity. There was a significant relation between &Dgr;POP before volume expansion and percent change in cardiac index after volume expansion (r = 0.62; P < 0.05). Conclusions:&Dgr;POP can predict fluid responsiveness noninvasively in mechanically ventilated patients during general anesthesia. This index has potential clinical applications.

Rheology of complex fluids by particle image velocimetry in microchannels

We image the flow of complex fluids in microchannels of controlled geometry using tracers. The spatial resolution allows us to access quantitatively the bulk nonlinear rheology and wall slip, as we show on model polymer solutions. In perspective this strategy should prove useful for the study of heterogeneous flows of more complex fluids.

Capillary filling in closed end nanochannels

We investigated the interactions between liquid, gas, and solid phases in the capillary filling process of closed-end nanochannels. This paper presents theoretical models without and with absorption and diffusion of gas molecules in the liquid. Capillary filling experiments were carried out in closed-end silicon nanochannels with different lengths. The theoretical and measured characteristics of filling length versus time are compared. The results show that the filling process consists of two stages. The first stage resembles the capillary filling process in an open-end nanochannel. However, a remarkable discrepancy between the experimental results and the theory without gas absorption is observed in the second stage. A closer investigation of the second stage reveals that the dissolution of gas in the liquid is important and can be explained by the model with gas absorption and diffusion.

Amplification of electro-osmotic flows by wall slippage: direct measurements on OTS-surfaces

The control of water flow in Electrostatic Double Layers (EDL) close to charged surfaces in solution is an important issue with the emergence of nanofluidic devices. We compare here the zeta potential governing the electrokinetic transport properties of surfaces, to the electrostatic potential directly measured from their interaction forces. We show that on smooth hydrophilic silica these quantities are similar, whereas on OTS-silanized hydrophobic surfaces the zeta potential is significantly higher, leading to an enhanced electro-osmotic velocity. The enhancement obtained is consistent with an interfacial water slippage on the silanized surface, characterized by a constant slip length of approximately 8 nm independent of the salt concentration in the range 10(-4)-10(-3)M.

Roles of gas in capillary filling of nanoslits

Control and understanding of flows inside fabricated nanochannels is rich in potential applications, but nanoscale physics of fluids remains to be clarified even for the simple case of spontaneous capillary filling. This paper reports an experimental and modelling investigation of the role of gas on the capillary filling kinetics slowdown in nanoslits (depth going from 20 nm to 400 nm) compared to Washburns prediction. First, the role of gas through the usually observed trapped bubbles during a nanoslits capillary filling is analysed thanks to experiments realized with water, ethanol and silicone oil in silicon-glass nanochannels. Bubbles are trapped only when slit depth is below a liquid-dependent threshold. This is interpreted as possible contact line pinning strength varying with wettability. Stagnant trapped bubbles lifetime is investigated for the three liquids used. Experimental results show that bubbles are first compressed because of the increasing local liquid pressure. Once the gas bubble pressure is sufficiently high, gas dissolution induces the final bubble collapse. Influence of the bubbles’ presence on the capillary filling kinetics is analysed by estimating viscous resistance induced by the bubbles using an effective medium approach (Brinkman approximation). Surprisingly, the bubbles’ presence is found to have a very minor effect on nanoslits capillary filling kinetics. Second, the transient gas pressure profile between the advancing meniscus and the channel exit is computed numerically taking into account gas compressibility. A non-negligible over-pressure ahead of the meniscus is found for nano-scale slit capillary filling. Considering the possible presence of precursor films, reducing cross-section for gas flow, leads to a capillary filling kinetics slowdown comparable to the ones measured experimentally.

Complications digestives sévères après chirurgie cardiaque sous circulation extracorporelle

ObjectifÉvaluer l’incidence, les modalités de survenue et l’évolution des complications digestives après chirurgie cardiaque sous circulation extracorporelle (CEC).MéthodeÉtude rétrospective sur 6281 adultes opérés sous CEC entre le 1er janvier 1994 et le 31 décembre 1997.RésultatsSoixante patients ont présenté 68 complications digestives (1 %): hémorragie digestive haute (n = 23), ischémie intestinale (n = 19), cholécystite aiguë (n = 7), pancréatite aiguë (n = 6), dilatation colique (n = 13). L’incidence de ces complications, faible après chirurgie coronaire (0,4 %) ou valvulaire (0,8 %), était élevée après transplantation cardiaque (6 %) ou chirurgie d’une dissection aortique (9 %). Comparés à une population témoins, les patients ayant présenté une complication digestive avaient un score de Parsonnet plus élevé (29 ± 15 contre 13 ± 12 points, P = 0,002), étaient plus souvent opérés en urgence (40/60, 66 % contre 1120/6221, 18 %; P = 0,01), avaient subi une CEC plus longue (114 ± 66 contre 74 ± 42 min, P = 0,01), et avaient présenté plus fréquement un bas débit cardiaque postopératoire (45/60, 75 % contre 435/6221, 7 %; P = 0,001). La mortalité globale en présence d’une complication digestive a été de 52 %. Les facteurs associés à la mortalité étaient: survenue d’un sepsis (OR=38,7), survenue d’une insuffisance rénale (OR=7,9), âge > 75 ans (OR= 3,5), ventilation mécanique > 7 jours (OR=2,7), association d’une complication neurologique (OR=3,9).ConclusionLes complications digestives après CEC surviennent chez une population à risque. Ces complications s’intègrent dans un contexte de défaillance multiviscérale à l’origine d’une mortalité élevée.AbstractPurposeTo determine the incidence, circumstances of occurrence and evolution of gastrointestinal complications after cardiac surgery with extracorporeal circulation (ECC).MethodsRetrospective chart study of gastrointestinal complications in 6,281 patients undergoing ECC between January 1994 and December 1997.ResultsSixty patients developed 68 gastrointestinal complications (1%). Complications included: upper gastrointestinal bleeding (n = 23), intestinal ischemia (n = 19), cholecystitis (n = 7), pancreatitis (n = 6), and paralytic ileus (n = 16). The incidence of these complications was low after coronary artery (0.4%) or valvular surgery (0.8%) and high after cardiac transplantation (6%) and after surgery for acute aortic dissection (9%). Compared with a control population, patients with gastrointestinal complication had a higher Parsonnet score (29 ± 15 vs 13 ± 12 points; P = 0.002), were more frequently operated upon as an emergency (40/60, 66% vs 1120/6221, 18%; P = 0.01), underwent ECC of longer duration (114 ± 66 vs 74 ± 42 min; P = 0.01), and presented more frequently with low cardiac output after surgery (45/60, 75% vs 435/6221, 7%; P = 0.001). The mortality rate after gastrointestinal complications was 52%. The major factor associated with mortality was the occurence of sepsis (OR=38.7). Other factors were: renal failure (OR=7.9), age > 75 yr (OR=3.5), mechanical ventilation for more than seven days (OR=2.7), associated cerebral damage (OR=3.9).ConclusionGastrointestinal complications after ECC occur in high risk surgical patients. These complications are frequently associated with other complications leading to a high mortality rate.

DNA separation and enrichment using electro-hydrodynamic bidirectional flows in viscoelastic liquids

DNA size separation followed by purification and enrichment constitute essential operations for genetic engineering. These processes are mostly carried out using DNA electrophoresis in gels or in polymer solutions, a well-established yet lengthy technique which has been notably improved using Lab-on-Chip technologies. So far, innovations for DNA separation or enrichment have been mostly undertaken separately, and we present an approach that allows us to perform these two processes simultaneously for DNA fragments spanning 0.2-50 kilo base pairs (kbp) in length. Our technology involves an electric field and a counter hydrodynamic flow in viscoelastic liquids, in which we show the occurrence of transverse forces oriented toward the walls. These forces increase with DNA molecular weight (MW) and hence induce a progressive reduction in DNA migration speed that triggers size separation in microfluidic channels as well as in capillaries. The separation of MW markers in the range 1-50 kbp is achieved in 15 minutes, thus outperforming gel electrophoresis that takes ∼3 hours for this sample. Furthermore, the use of a funnel, where electric and flow fields are modulated spatially, enables us to adjust the transverse forces so as to stall the motion of DNA molecules at a position where they accumulate at factors of up to 1000 per minute. In this configuration, we establish that the operations of DNA enrichment and separation can be carried out simultaneously for the bands of a DNA MW marker between 0.2-1.5 kbp diluted at 0.02 ng μL(-1) in 30 s. Altogether, our technology, which can readily be integrated as an in-line module in Lab-on-Chips, offers unique opportunities for sample preparation and analysis of minute genomic samples.