Pattern formation without heating in an evaporative convection experiment

We present an evaporation experiment in a single fluid layer. When latent heat associated to the evaporation is large enough, the heat flow through the free surface of the layer generates temperature gradients that can destabilize the conductive motionless state giving rise to convective cellular structures without any external heating. The sequence of convective patterns obtained here without heating, is similar to that obtained in B\'enard-Marangoni convection. This work present the sequence of spatial bifurcations as a function of the layer depth. The transition between square to hexagonal pattern, known from non-evaporative experiments, is obtained here with a similar change in wavelength.Comment: Submitted to Europhysics Letter

Convective Motion in a Vibrated Granular Layer

Experimental results are presented for a vertically shaken granular layer. In the range of accelerations explored, the layer develops a convective motion in the form of one or more rolls. The velocity of the grains near the wall has been measured. It grows linearly with the acceleration, then the growing rate slows down. A rescaling with the amplitude of the wall velocity and the height of the granular layer makes all data collapse in a single curve. This can provide insights on the mechanism driving the motion.Comment: 10 pages, 5 figures submitted to Phys. Rev. Let

Wettability of partially suspended graphene

The dependence of the wettability of graphene on the nature of the underlying substrate remains only partially understood. Here, we systematically investigate the role of liquid-substrate interactions on the wettability of graphene by varying the area fraction of suspended graphene from 0 to 95% by means of nanotextured substrates. We find that completely suspended graphene exhibits the highest water contact angle (85° ± 5°) compared to partially suspended or supported graphene, regardless of the hydrophobicity (hydrophilicity) of the substrate. Further, 80% of the long-range water-substrate interactions are screened by the graphene monolayer, the wettability of which is primarily determined by short-range graphene-liquid interactions. By its well-defined chemical and geometrical properties, supported graphene therefore provides a model system to elucidate the relative contribution of short and long range interactions to the macroscopic contact angle

Dynamics of nanodroplets on topographically structured substrates

Mesoscopic hydrodynamic equations are solved to investigate the dynamics of nanodroplets positioned near a topographic step of the supporting substrate. Our results show that the dynamics depends on the characteristic length scales of the system given by the height of the step and the size of the nanodroplets as well as on the constituting substances of both the nanodroplets and the substrate. The lateral motion of nanodroplets far from the step can be described well in terms of a power law of the distance from the step. In general the direction of the motion depends on the details of the effective laterally varying intermolecular forces. But for nanodroplets positioned far from the step it is solely given by the sign of the Hamaker constant of the system. Moreover, our study reveals that the steps always act as a barrier for transporting liquid droplets from one side of the step to the other.Comment: 44 pages, 25 figure

Drawing a single nanofibre over hundreds of microns

We demonstrate that the limits of drawing of fibre can be pushed down to nanometre dimensions. By a process similar to dry-spinning, nanofibres can be drawn with ultimate dimensions comparable with the ones of single-wall carbon nanotubes. An important difference with tubes is that our nanofibres are produced one at a time. It is also shown that the nanofibre can be positionned precisely on a surface during fabrication, and can then be cut or manipulated by an AFM tip

Dynamics of spreading of a liquid drop across a surface chemical discontinuity

We report an experimental study of the dynamics of a liquid ridge straddling a chemical discontinuity which separates two different solid surfaces. Measuring the displacements and contact angles on both sides, we describe in detail the different regimes of motion and in particular a stationary regime which corresponds to uniform translation. These results are in qualitative agreement with a theoretical model, although we see a deformation of the drop shape when compared to a circular profile, which is the currently admitted hypothesis.Nous présentons une étude expérimentale de la dynamique d'étalement d'un ruban liquide déposé à cheval sur la discontinuité chimique qui sépare deux surfaces solides différentes. Les mesures de déplacements et d'angles de contact sur les deux bords du ruban permettent de décrire en détail les différentes phases du mouvement et en particulier un régime stationnaire correspondant à une translation uniforme. Ces résultats sont en accord qualitatif avec un modèle théorique, bien qu'ils mettent en évidence une déformation du ruban par rapport au profil circulaire constituant l'hypothèse couramment admise

Ancrage d’un liquide sur une discontinuité chimique de surface

Nous étudions par la méthode de Wilhelmy le comportement d'un front liquide sur la ligne de discontinuité chimique séparant deux substrats de mouillabilités différentes. Deux comportements opposés sont observés et discutés : lorsque le liquide recouvre la partie hydrophile, la ligne de contact reste ancrée sur la frontière pour un intervalle de valeurs de l'angle de contact que l'on déterminera; au contraire, si le liquide est sur la partie hydrophobe, la ligne de contact est en position instable sur la frontière

Co-planar carbon nanotube hybrid molecular transistors fabricated in parallel

We show that a molecular combing process can be used to fabricate in parallel, in one step, a large number of co-planar carbon nanotube hybrid molecular transistors. The gate voltage is applied by the electrodes of a nanojunction thus increasing the miniaturisation of the device. We show with such devices that the bias coplanar grid is not very active on metal-like nanotubes and that the same conductance change can be obtained by deforming the tube by the tip of an atomic force microscope. On the contrary, the grid is very active on a semiconductor-like tube leading to transconductances of 160 nA/V