PubMed2005
In this work a generalized hydrodynamic theory for water flow into a mesoporous matrix from hydrophobized silica gel is suggested. Although we examine a fluid dynamics problem, the motion of the water-gas-solid contact line past a hydrophobized silica gel surface, motivation for such research derives from the investigation of a novel principle of mechanical energy dissipation, called surface dissipation, and its attached machine element, named a colloidal damper (CD). Similar to a hydraulic damper, this absorber has a cylinder-piston structure, but oil is replaced by a colloid consisting of a mesoporous matrix and a lyophobic liquid. Here, the mesoporous matrix is from silica gel modified by linear chains of alkyldimethylchlorosilanes and water is the associated lyophobic liquid. Mainly, the colloidal damper energy loss can be explained by the dynamic contact angle hysteresis in advancing (liquid displaces gas) and receding motion (gas displaces liquid); such hysteresis occurs due to the geometrical and chemical heterogeneities of the solid surface. Although this new kind of dissipation could be attractive for many applications, the subject remains almost unexplored in the scientific literature. Many different, complex, and interconnected aspects are related to this subject: capillary hydrodynamics, slippage effect, contact angle hysteresis, estimation of dissipated energy, thickness optimization of the grafted layer on the surface of the mesoporous matrix, etc. For this reason, a novel and global approach to all the complex and interconnected phenomena related to the contact line movement past a solid surface from hydrophobized silica gel is proposed. Our approach has a modest experimental basis but this is compensated for with rich references to other experimental and theoretical work oriented to the study of surface phenomena in such systems. We tried to sort the existing results and to find the right place for each in building our global view of the problem. This work is structured as follows. The measurement technique of the hysteresis loop is described. From experimental data one calculates the dissipated energy versus length of the grafted molecule on the silica gel surface. These results are justified by flow analysis. Generalized hydrodynamic theory means here that the basic structure of the Navier-Stokes equations is kept, but in order to include the relation between macroscopic flow and molecular interactions, slip is allowed on the solid wall. The nanopillar architecture of the silica gel hydrophobic coating is described. Concepts of slip and contact angle hysteresis are detailed and their connection is revealed. During adsorption, water penetrates the pore space by maintaining contact with the top of the coating molecules (region of -CH(3) groups); after that, water is forced into and partially or totally fills the space between molecules (region of -CH(2) groups). In such circumstances, at the release of the external pressure, desorption occurs. An original energetic-barriers approach is proposed to understand the filling of the nanosize canals which occur in the hydrophobic grafted layer. Employing this energetic-barriers approach, one finds the optimum length of the grafted molecule which maximizes the dissipated energy of the CD reversible cycle. Such results are useful for the appropriate design of ultrahydrophobic surfaces in general, and for the optimal design of a hydrophobic coating of a mesoporous matrix destined for CD use.