Spreading of nanofluids on solids
Entry by Richie Tay for AP 225 Fall 2012
Authors: Darsh Wasan, Alex Nikolov
Nanofluids are dilute suspensions of nanoparticulates (e.g. micelles, globular proteins, metal particles). They are widely investigated for their enhanced thermophysical properties , but their unique spreading behavior also makes them potentially useful for soil remediation, lubrication, and oil recovery, among other applications . Unlike simple liquids, whose spreading velocity scales inversely with viscosity, nanofluids show enhanced spreading as nanoparticle concentration (and hence liquid viscosity) is increased . Theoretical calculations suggest that this phenomenon could arise from organization of the nanoparticles in confined spaces; in this paper, the authors sought experimental evidence of this ordering using video microscopy.
Results and Discussion
To examine particle structuring, the authors started with a nanofluid comprising 7 vol.% charged latex spheres in deionized water. They trapped a 200μm-diameter air bubble under a glass plate in the nanofluid, and looked at the wedge-like three-phase contact region using reflected-light digital video microscopy (Fig. 1a). They found that the particles formed a 2D colloidal crystal at a wedge-film thickness twice the particle diameter, but beyond three particle diameters, the particles became disordered. Figure 1b shows the particles in the wedge region.
The observed particle distribution pattern (Fig. 2, left) corresponded to calculations of the disjoining pressure <math>\Pi</math> (Fig. 2, middle), as well as to computer simulations. The oscillatory pattern arose from changes in particle-particle mean interaction potential as crystallization occurred in the wedge films. Also shown in Figure 2 (right) is the spreading coefficient S (estimated using and theoretical calculations for <math>\Pi</math>), which increased as film thickness decreased. The sharp change in the slope for S occured at the wedge thickness where there was particle ordering, thus proving that this structuring was what enhanced the spreading of nanofluids on solids.
As a demonstration of the utility of this atypical spreading, the authors then simulated an oily soil removal process using an oil droplet immersed in an aqueous suspension of SDS (0.4 vol.%, 10x the critical micelle concentration). With the aid of differential and common three-phase contact angle interferometry, they showed the sequential penetration of the nanofluid between the oil and glass surface, leading to the formation of small aqueous lenses (white spots encircled by dark fringes) and the eventual separation of the oil drop from the glass by a thick aqueous film with a dimple (Fig. 3). They hypothesized that, as in the earlier experiment, micelle diffusion into the wedge film was responsible for setting up a disjoining pressure gradient that facilitated spreading of the micellar solution across the solid surface.
By adding an electrolyte (0.1M NaCl) to the nanofluid, detachment of the oil drop was prevented. This might appear counterintuitive, since the interfacial tension at the interface between the oil and the nanofluid decreased at a higher salt concentration, which caused the drop to shrink and should have enhanced the separation process. The authors believed that the high salt concentration resulted in a decrease in the effective micelle diameter (and hence micelle volume fraction) due to shrinkage of the electrical double layer around each micelle. This could have been sufficient to reduce the disjoining pressure gradient and the driving force for drop detachment.
This article is interesting because it showed empirically that particle structuring near a three-phase contact region is able to set up a disjoining pressure gradient that influences spreading of a liquid in that region. This is unlike simple fluids, where the spreading coefficient S is independent of film thickness; in nanofluids, S depends on the thickness of the liquid film, which affects organization of nanoparticles in that film.