Convective Drying of Films of Polymer Solutions: Front Propagation Revealed by Thermal Measurements.

We report on the drying of films of polymer solutions under a controlled laminar air flow. Temperature measurements reveal that a drying front propagates in the film at constant velocity. Using thermal calibration, we are able to quantitatively determine the local drying rate of the film, and we find it agrees with conservation arguments.

Digital Microfluidics─How Magnetically Driven Orientation of Pillars Influences Droplet Positioning.

Interfaces between a water droplet and a network of pillars produce eventually superhydrophobic, self-cleaning properties. Considering the surface fraction of the surface in interaction with water, it is possible to tune precisely the contact angle hysteresis (CAH) to low values, which is at the origin of the poor adhesion of water droplets, inducing their high mobility on such a surface. However, if one wants to move and position a droplet, the lower the CAH, the less precise will be the positioning on the surface.

Drying Liquid Coatings with an Evaporation Mask: Theory and Experiments.

We report a study of the spatially varying thickness of dried films of polymer solutions resulting from a nonuniform evaporation flux. The controlled heterogeneity of the evaporation flux is imposed by placing a solid mask above the evaporating film spread on a solid substrate. At the end of drying, a depression has formed under the mask, together with overthicknesses extending from the edge of the mask and over distances that may be larger than its size.

Towards a passive limitation of particle surface contamination in the Columbus module (ISS) during the MATISS experiment of the Proxima Mission.

Future long-duration human spaceflight calls for developments to limit biocontamination of the surface habitats. The MATISS experiment tests surface treatments in the ISS's atmosphere. Four sample holders were mounted with glass lamella with hydrophobic coatings, and exposed in the Columbus module for ~6 months.

Experimental studies of the transmission of light through low-coverage regular or random arrays of silica micropillars supported by a glass substrate.

The transmission of light through low-coverage regular and random arrays of glass-supported silica micropillars of diameters 10-40 µm and height 10 µm is studied experimentally. Angle-resolved measurements of the transmitted intensity are performed at visible wavelengths by either a goniospectrophotometer or a multimodal imaging (Mueller) polarimetric microscope. It is demonstrated that for the regular arrays, the angle-resolved measurements are capable of resolving many of the densely packed diffraction orders that are expected for periodic structures of lattice constants 20-80 µm, but they also display features ("halos" and fringes) that are due to the scattering and guiding of light in individual micropillars or in the supporting glass slides.

New Platform for Gravitational Microfluidic Using Ferrofluids.

Among the large variety of microfluidic platforms, surface devices are a world apart. Electrowetting systems are used to control the displacement of droplets among predetermined pathways. More confidential, superhydrophobic surfaces are more and more described as new elements to guide spherical droplet reactors.

Swelling Dynamics of Surface-Attached Hydrogel Thin Films in Vapor Flows.

Hydrogel coatings absorb water vapor, or other solvents, and, as such, are good candidates for antifog applications. In the present study, the transfer of vapor from the atmosphere to hydrogel thin films is measured in a situation where water vapor flows alongside the coating which is set to a temperature lower than the ambient temperature. The effect of the physico-chemistry of the hydrogel film on the swelling kinetics is particularly investigated.

Wetting against the nap - how asperity inclination determines unidirectional spreading.

We have carried out wetting experiments on textured surfaces with high aspect ratio asperities in the Wenzel state. When inclination is imparted to the asperities, we observe a strictly unidirectional spreading opposite to the direction in which the asperities point. The advancing contact angle decreases markedly as inclination increases.

Self-replicating cracks: a collaborative fracture mode in thin films.

Straight cracks are observed in thin coatings under residual tensile stress, resulting into the classical network pattern observed in china crockery, old paintings, or dry mud. Here, we present a novel fracture mechanism where delamination and propagation occur simultaneously, leading to the spontaneous self-replication of an initial template. Surprisingly, this mechanism is active below the standard critical tensile load for channel cracks and selects a robust interaction length scale on the order of 30 times the film thickness.

Finite size effects on textured surfaces: recovering contact angles from vagarious drop edges.

A clue to understand wetting hysteresis on superhydrophobic surfaces is the relation between receding contact angle and surface textures. When the surface textures are large, there is a significant distribution of local contact angles around the drop. As seen from the cross section, the apparent contact angle oscillates as the triple line recedes.

Controlled angular redirection of light via nanoimprinted disordered gratings.

Enhanced control of diffraction through transparent substrates is achieved via disordered gratings in a silica sol-gel film. Tailoring the degree of disorder allows tuning of the diffractive behavior from discrete orders into broad distributions over large angular range. Gratings of optical quality are formed by silica sol-gel nanoimprint lithography and an optical setup for the measurement of continuous diffraction patterns is presented.

Role of kinks in the dynamics of contact lines receding on superhydrophobic surfaces.

We have investigated the depinning of the contact line on superhydrophobic surfaces with anisotropic periodic textures. By direct observation of the contact line conformation, we show that the mobility is mediated by kink defects. Full 3D simulations of the shape of the liquid surface near the solid confirm that kinks account for the measured wetting properties.

All-silica nanofluidic devices for DNA-analysis fabricated by imprint of sol-gel silica with silicon stamp.

We present a simple and cheap method for fabrication of silica nanofluidic devices for single-molecule studies. By imprinting sol-gel materials with a multi-level stamp comprising micro- and nanofeatures, channels of different depth are produced in a single process step. Calcination of the imprinted hybrid sol-gel material produces purely inorganic silica, which has very low autofluorescence and can be fusion bonded to a glass lid.