Fluctuation Forces and Wetting Layers in Colloid-Polymer Mixtures

Abstract

We present confocal microscopy experiments on the wetting of phase-separated colloid-polymer mixtures. We observe that an unusually thick wetting layer of the colloid-rich phase forms at the walls of the glass container that holds the mixture. Because of the ultralow interfacial tension between the colloid-rich and the polymer-rich phases, the thermally activated roughness of the interfaces becomes very big and measurable. We observe that close to the critical point the roughness of the interface between the wetting layer and the polymer-rich phase decreases with decreasing layer thickness: large excursions of the interface are confined in the wetting layer. The measured relationship between the roughness and the thickness of the wetting layer is in qualitative agreement with the predictions of renormalization group theory for short-range forces and complete wetting. DOI: 10.1103/PhysRevLett.100.178305 PACS numbers: 82.70.Dd, 68.08.Bc, 68.35.Ct, 68.37.ÿd Wetting layers are ubiquitous in both nature and industry [1,2] and occur in a wide range of systems. For two coexisting phases, Cahn first proposed an argument that explains the formation of a wetting layer of one of the two phases onto a substrate (complete wetting) when approaching the critical point [3]. One classical example is a demixed (two-phase) binary liquid mixture in which the dense phase intrudes between the lighter fluid phase and the vapor. However, once the wetting conditions are set, the remarkable density inversion can only be maintained if forces act between the two film interfaces. For ordinary liquids, these forces are generally van der Waals forces [1,2] which, given their range, invariably lead to wetting layer thicknesses of the order of 100 Å .Liquid interfaces are also rough-they are locally fluctuating due to thermally excited capillary waves [4]. Consequently, the confinement of interface fluctuations in a wetting layer can in principle create an entropic interaction that enters the force balance between surface forces and gravity [5]. The existence of such an entropic force was first established by Helfrich [6] in the case of tensionless membranes. For liquid interfaces, where surface tension dominates, the situation is more complex. From a theoretical point of view, the influence of capillary fluctuations on wetting phenomena has attracted much attention as it could lead to nonuniversal wetting behavior with critical exponents depending explicitly on temperature [7,8]. Experimentally, the observation of this entropic repulsion in typical liquid layers is challenging, but some observations have shown that a correct description of adsorption isotherms of molecularly thin films could be achieved if one takes the entropic interaction into account [9][10][11][12].Similarly to simple liquids, mixtures of colloidal particles with nonadsorbing polymer can phase separate into a dense colloidal fluid phase (colloidal liquid) and a dilute one (colloidal gas) [13]. This phase separation is due to the depletion attraction that the polymer m...

Mesoscopic Hamiltonian for the fluctuations of adsorbed Lennard-Jones liquid films

“…The experimental validation [15][16][17] of mesoscopic Hamiltonian models has to discriminate these subtle effects from the simple local Hamiltonian Eq. (2).…”

Rounding of Phase Transitions in Cylindrical Pores

“…The latter system is attractive for experiments: the large colloid size renders effects of the atomistic corrugation of pore walls negligible, and facilitates observation of wetting layers and interfaces [38]. We describe colloids as hard spheres of radius r c = 1, and polymers as soft spheres of radius r p = 0.8.…”

Fluid–fluid coexistence in an athermal colloid–polymer mixture: thermodynamic perturbation theory and continuum molecular-dynamics simulation

“…Experimental investigations of the phase behaviour of colloid-polymer mixtures have been performed on a variety of systems ranging from comparatively simple latex-polystyrene and silica-polydimethylsiloxane colloidal dispersions to biological systems containing proteins or DNA [2][3][4][5][6][7][8][9][10][11][12][13]. Phase separation has been described by many authors [3,4,8,[14][15][16][17][18][19][20][21][22][23][24][25][26][27]; see the excellent book of Lekkerkerker and Tuinier [1] for a comprehensive historical review. Fluid-solid coexistence is observed when the radius of gyration of the polymer is small compared to that of the colloid; here one can draw an analogy between the behaviour of the colloids and that of a system of pure hard spheres (HSs).…”