An innovative method to control the incipient flow boiling through grafted surfaces with chemical patterns.

The onset of flow boiling of a liquid is linked to the superheat condition that is necessary to activate the nucleation sites on contacting surfaces. The nucleation sites are usually represented by cavities in the rough surface of the heat exchanger. On smooth surfaces, the region where bubble detachment does not occur due to the lack of superheating may constitute a serious limitation for microfluidic devices. This paper shows the first experimental evidence that the position of the active nucleation sites can be controlled through chemical patterning of smooth surfaces: in this study, the heated surfaces are chemically grafted with alkylsilane self-assembled monolayers by microcontact printing. The analysis of the propagation of the bubble zone area quantitatively shows that the bubbles remain localized on top of the grafted zone and that, in the initial phase of the experiment, the center of mass of the bubble zone only moves along the vertical axis, without lateral drift.

Fourier transform infrared immunosensors for model hapten molecules.

We report experimental results concerning the detection of 2,4-dinitrophenol, under its free form or coupled to human serum albumin using Fourier transform infrared spectroscopy-based sensors. Competitive immunoreactions were carried out using several anti-dinitrophenol monoclonal antibodies. Comparison with enzyme-linked immunosorbent assays in competition is given for standard operating conditions. FTIR detection limits are comparable to those obtained by ELISA. The limits of detection are about 5-15 ng/mL for the coupled DNP. Using the LO-DNP61 antibody, a detection limit of congruent with 5 ng/mL was also estimated for the free DNP molecules but is much higher for the other antibodies.

Ligand-receptor interactions in complex media: a new type of biosensors for the detection of coagulation factor VIII.

Detection of receptor-ligand interaction in complex media remains a challenging issue. We report experimental results demonstrating the specific detection of the coagulation factor VIII in the presence of a large excess of other proteins using the new BIA-ATR technology based on attenuated total reflection Fourier transform infrared spectroscopy. The principle of the detection is related to the ability of factor VIII molecules to bind to lipid membranes containing at least 8% phosphatidylserine. Several therapeutic concentrates of factor VIII were analyzed and the binding of the coagulation factor was monitored as a function of time. We show that a non-specific adsorption of stabilizing agents (typically, von Willebrand factor and human serum albumin) may be avoided by controlling the geometry of the ATR element. A linear response of the sensors as a function of the factor VIII concentration is described for different lipid membrane compositions.

Superhydrophobic surfaces from various polypropylenes.

Superhydrophobic surfaces were prepared from solutions of isotactic polypropylenes of various molecular weights using soft chemistry. Varying the conditions of the experiments (polymer concentration and initial amount of the coated solution) allowed us to optimize the superhydrophobic behavior of the polymer film. Results show that decreasing the concentration and/or film thicknesses decreases the probability to get superhydrophobicity for all polypropylenes tested. Measurement and analysis of advancing and receding contact angles as well as estimation of surface homogeneity were performed. Similar results were obtained with syndio- as well as atactic polypropylenes.

Drop impact on porous superhydrophobic polymer surfaces.

Water drop impacts are performed on porous-like superhydrophobic surfaces. We investigate the influence of the drop size and of the impact velocity on the event. The Cassie-Baxter/Wenzel transition is observed to be a function of the drop size, as well as the outcomes of the impact or deposition process, which can be deposition, rebound, sticking, or fragmentation. A quantitative analysis on the experimental conditions required to observe rebound is provided. Our analysis shows that the wettability hysteresis controls the limit between deposition and rebound events. This limit corresponds to a constant Weber number. A survey of literature results on impact over patterned superhydrophobic surfaces is provided as a comparison.

Experimental investigation of the link between static and dynamic wetting by forced wetting of nylon filament.

Forced wetting experiments with various liquids were conducted to study the dynamic wetting properties of nylon filament. The molecular-kinetic theory of wetting (MKT) was used to interpret the dynamic contact angle data and evaluate the contact-line friction zeta0 at the microscopic scale. By taking account of the viscosity of the liquid, zeta0 could be related exponentially to the reversible work of adhesion. This clearly establishes an experimental link between the static and dynamic wetting properties of the material. Moreover, statistical analysis of the equilibrium molecular displacement frequency K0 and the length of the displacements lambda reveals that these two fundamental parameters of the MKT are strongly correlated, not only in the linearized form of the theory (valid close to equilibrium) but also when the nonlinear form of the equations has to be considered at higher wetting speeds.

Contact-line friction of liquid drops on self-assembled monolayers: chain-length effects.

The static and dynamic wetting properties of self-assembled alkanethiol monolayers of increasing chain length were studied. The molecular-kinetic theory of wetting was used to interpret the dynamic contact angle data and evaluate the contact-line friction on the microscopic scale. Although the surfaces had a similar static wettability, the coefficient of contact-line friction zeta0 increased linearly with alkyl chain length. This result supports the hypothesis of energy dissipation due to a local deformation of the nanometer-thick layer at the contact line.

Biochemical interaction analysis on ATR devices: a wet chemistry approach for surface functionalization.

A new generic device suitable for the investigation of ligand-receptor interactions is presented. In particular, the research focused on optical waveguides constituted by an attenuated total internal reflection (ATR) element, transparent in the infrared and whose surfaces were activated in view of covalently binding a receptor. Silicon and germanium ATR elements were considered. The original method is based on the grafting of bifunctional spacer molecules directly at the surface of the germanium crystal, avoiding the deposition of an intermediate metal layer. The grafting of these binding molecules (under their N-hydroxysuccinimidyl ester forms) was performed either by wet chemistry or by photochemistry. The functionalized surfaces, which allow the binding of molecules bearing peripherical NH2 groups, were successfully used, e.g., for the detection of proteins (streptavidin) or of small molecules (biotin). In the latter case, the biotin was readily detected for concentrations as low as 10(-12) M.

Site-specific counterion binding: application of the standard Poisson-Boltzmann cell model to ionic polysaccharides of the plant cell wall.

A numerical method is presented for analysing the effects of the competitive processes of dissociation and complexation that occur at the level of the functional groups of charged polysaccharides during ion exchange experiments carried out on plant cell walls. The interactions between the exchange sites and the mobile ions are handled by the standard Poisson-Boltzmann cell model. The dissociation and the complexation are described by mass-action laws. Uncomplexed counterions are involved in the formation of a cylindrical double layer. This latter type of interaction determines the dissociation degree of the polyion. A least-squares algorithm is used to evaluate the complexation constants of the counterions from experimental data. The need of introduction of the complexation constant of the divalent counterions has been clarified by a comparison between theoretical titration curves calculated with and without specific site binding. The fraction of complexed sites does not agree with that predicted from Oosawa's theory of the condensation. The accumulation coefficients, calculated from the optimised theoretical titration curves, clearly demonstrate the competition between the protonation and the complexation processes at the level of the pectic acids of the wall.