Here, with the aim of obtaining densely packed porous nanostructures of various shape using templateless electropolymerization in organic solvent (dichloromethane), original thieno[3,4- b ]thiophene-based monomers with different substituents are studied. First of all, the adding of water in solution has a huge influence on the formation of porous structures because a much higher amount of gas (O 2 and/or H 2 ) is released. Rigid substituents such as aromatic groups have a beneficial effect on the formation of nanotubular structures contrary to flexible ones such as alkyl chains. Special results are obtained with the pyrene substituent (Thieno-Pyr). With this monomer, coral-like structures are obtained. These structures are obtained by the formation first on long nanotubular structures and their sagging due to their weight. Then, the released gas is trapped inside these structures leading to huge craters. These exceptional surfaces could be used in the future in various potential applications such as in drug delivery, cell growth, sensors, optical devices or surface adhesion. This article is part of the theme issue ‘Bioinspired materials and surfaces for green science and technology (part 2)’.

Natural convection heat transfer in open or closed cavities takes place in different engineering areas. The hemispherical cavity is a part of basic geometries although it is not widely studied. The present paper reports the numerical study of natural convection in a closed hemispherical annulus delimited by two vertically eccentric hemispheres filled with Newtonian fluid (air in this case with Pr = 0.7) is conducted. The inner hemisphere is heated by a heat flux of constant density and the outer one is maintained isothermal. Based on the Boussinesq assumptions, the governing equations are numerically studied using unsteady natural convection formulated with vorticity and stream-function variables. These equations are written by using bispherical coordinates system and solved by using a finite difference method. The effect of the control parameters such as the Rayleigh number ( 3 6 10 10 Ra ≤ ≤ ) or the eccentricity (e = ±0.2, ±0.5, 0) in the dynamic and thermal behaviours of the fluid is investigated.

Here, we control the surface hydrophobicity and the adhesion of water droplets by electrodeposition of poly(3,4-ethylenedioxypyrrole) (PEDOP) and poly(3,4-propylenedioxypyrrole) (PProDOP) with branched alkyl chains placed preferentially on the bridge to favor the formation of nanofibers. Branched alkyl chains of various sizes from very short (C 3 ) to hyperbranched (C 18 ) are studied because they have lower surface hydrophobicity than long alkyl or fluoroalkyl chains (preferable for parahydrophobic properties). The electrodeposition is much more favored with the PEDOP derivatives because the ProDOP films are more soluble. However, the formation of nanoparticles is favored with the PEDOP polymers in contrast to the formation of fibers, resembling the wax nanoclusters observed on lotus leaves, with the ProDOP polymers. With both these PEDOP and PProDOP derivatives, it is possible to reach parahydrophobic properties characterized by a sticking behavior toward water droplets. This kind of surfaces could be used in the future in water harvesting systems, for example.

In this original work, we wanted to prepare surfaces with both high hydrophobicity and strong water adhesion, as observed on rose petals or gecko foot. The surfaces are prepared by electropolymerization and using thieno [3,4-b]thiophene monomers with various branched alkyl chains. It is observed a change from structured to smooth when the branched alkyl chain length increases. Using short branched alkyl chains, the surfaces are mainly composed of wrinkles and nanoparticles displaying high hydrophobicity (w up to 134.9°) combined with strong water adhesion, as observed on rose petals. Indeed, dynamic contact angle measurements show that the real water adhesion can be very different even if apparent contact angle is the same. By adding H2O in the solvent (CH2Cl2 + H2O), it is possible to release gas bubbles (H2 and/or O2) in-situ leading to more porous surfaces but here it is observed an increase in surface hydrophilicity. These original polymer surfaces have potential applications in water harvesting systems or oil/water separation membranes, for example.

This work is a contribution to the numerical study of the phenomenon of heat transfer by laminar natural convection of an electrically conductive Newtonian fluid subjected to a uniform horizontal magnetic field. The study focused on a hemispherical cavity delimited by two vertically eccentric hemispheres. A constant flux density is imposed on the inner hemisphere while the outer hemisphere is maintained at a constant temperature. The combination of thermal and electrical boundary conditions is exploited to obtain the critical values of the parameters marking the onset of instability. The Boussinesq approximation is used to study the equations governing this fluid instability. The projection of these equations in the bispheric coordinate system as well as the discretization by the finite difference method facilitated the development of a computer code in Fortran. The exploitation of this code made it possible to determine the growth rates for Hartmann values equal to 1; 10 and 100, from Rayleigh equal to 103; 104; 105 and 106, with eccentricity equal to ± 0.2; ± 0.5 and 0 and a radius ratio equal to 2. The aim is to highlight the effect of the magnetic field on the heat transfer. At the end of the study, the results obtained are consistent and revealing: they are in good agreement with those of references drawn from the literature.

In this article, we have numerically studied the phenomenon of laminar mixed convection in a ventilated square cavity. The vertical walls of the cavity are subjected to a temperature gradient while the horizontal walls are constrained as adiabatic. We used the finite volume method to discretize the system of non-dimensional equations and a purely implicit scheme for the temporal discretization. The results were presented in the form of hydrodynamic and thermal fields for different values ​​of the Richardson number with the number of Reynolds constant. We found that the increase of the Richardson number with a fixed Reynolds number Re=100 allows us to visualize the evolution of the system and to appreciate the efficiency of the bottom ventilation.

A numerical modeling of the effect of the ratio of thermal conductivity on the thin film condensation in forced convection in a canal whose walls are covered with a porous material is presented. In this work, the generalized Darcy-Brinkman-Forchheimer (DBF) equations in the porous medium and the hydrodynamic and thermal boundary layer equations in the pure liquid, were used. Rendered dimensionless and homotopically transformed into a new rectangular basis, we used a finite difference method to discretize them. The advection and the diffusion terms are discretized with respectively a backward-centered scheme and a centered scheme. After validation, we find that a variation of the longitudinal velocity as a function of the ratio of thermal conductivity only for low values of the Peclet number. When the ratio of thermal conductivity increases, corresponding to an increasingly conductive medium, the longitudinal velocity, the temperature and the Nusselt number increase (even when the Peclet number is high for the thermal field). While the thickness of the liquid film decreases (disadvantaged condensation) and leads to an increase in the length of entry, increase almost linear. The sensitivity of condensation to variations in the ratio of thermal conductivity is constant, whatever its value. The ratio of thermal conductivity is a very decisive and predictable physical quantity to properly examine the performance of condensation.

We have numerically studied the heat transfers in a Cylindrical-parabolic concentrator using a nanofluid as heat transfer fluid for its use in Solar Thermodynamics. After an analysis of the work relating to solar concentrators and nanofluids, we have, after having described our physical system and posed working hypotheses, written the equations which govern the transfers in our collector. The latter as well as the associated boundary conditions were then dimensionless in order to generalize the problem and reveal the parameters that control the operation of the absorber. To solve our equations, we used the method of finite differences and the algebraic system obtained is solved thanks to the Thomas method combined with an iterative process of line-by-line relaxation type. The computer code that we developed made it possible to find the temperature distributions according to the spatial coordinates and at different times. The effects and influences of wind effect, axial and transverse thermal dispersion, absorber length, geometric shape factor on the average temperature distributions of the coolant is analyzed. At the end of the study, we were able to identify the most important physical and geometric parameters which give our system optimum operation for its use in Solar Thermodynamics.