Detachment of colloids from a solid surface by a moving air–water interface
Graphical abstract
Introduction
Gas bubbles in a fluid can be used to remove particles from solid surfaces. When a gas bubble moves over a particle that is adhered to a solid surface, strong capillary forces form between the bubble and the particle, and the particle may detach from the adhering surface [1], [2]. This principle is used in industrial applications, for instance to clean silicon wafers [3].
Various chemical and physical parameters affect the efficiency of gas bubbles to detach particles from a solid surface. Busscher and coworkers used a horizontal parallel-plate flow chamber to study detachment of Latex particles from uncoated and coated quartz or microscope glass slides [1], [2], [4], [5]. They found that a moving liquid–air interface generates a very strong detachment force on adhered particles. The surface tension-based detachment force was several orders of magnitude larger than the adhesion force [1]. Particle detachment from surfaces by moving air-bubbles was more efficient for large liquid–air surface tensions and large particle sizes [2], [4], [5]. It was also observed that the more air-bubbles moved over a surface, the more particles were removed [2], [4].
That gas bubbles form strong capillary forces with particles at the gas–liquid–solid interface is known from theory, and forces have experimentally measured by atomic force microscopy [6], [7], [8]. The detachment process caused by air-bubbles involves interception, thinning of the liquid film, film rupture, formation of a three-phase line, and stabilization of particle–bubble aggregates [2], [9], [10]. A particle can attach to an air-bubble only when the particle–bubble contact time is larger than the induction time, that is the necessary time to thin the liquid film and form the three-phase contact line [10]. The interaction force between a bubble and a particle is strongly dependent on the particle–bubble contact angle. This dependency is used in flotation to separate suspended particles, where hydrophobic particles are preferentially removed by attachment to liquid–gas interfaces in form of bubbles raising to the surface of a liquid [2], [7], [10], [11].
Moving liquid–gas interfaces are also important for porous media flow and transport phenomena. It is likely that a moving liquid–gas interface can detach particles from porous media surfaces and carry particles along. In previous experiments, we have shown that a considerable amount of colloidal particles can be captured at the liquid–gas interface, and moved through a porous medium with an infiltration front [12]. Calculations using a numerical solution of the Young–Laplace equation have shown that subsurface colloids can be lifted from mineral surfaces by expanding water films [13]. From microscopic visualization using transparent micromodels, it is known that colloids can attach to the liquid–gas interfaces during transport through porous media [14], [15]. Sirivithayapakorn and Keller [16] observed that colloids (Latex particles) attach to the air–water interface and move with them, and colloids formed clusters when air-bubbles dissolved.
The effects of moving air-bubbles on the detachment of submicron-sized particles (usually Latex particles) from initially wet solid surfaces have been investigated under different physical and chemical conditions [1], [2], [3], [4], [5]. However, the effects of moving liquid–gas interfaces over initially dry surfaces have not yet been investigated. The movement of liquid–gas interfaces over initially dry surfaces occurs frequently in natural unsaturated porous media (e.g., the vadose zone), when water infiltrates or imbibes dry soil or sediments. In this work, we examined the detachment of colloids, attached to a solid surface under initially air-dry conditions, when the surface is wetted and a liquid–gas interface is moved over the colloids.
Our main objective was to study the effect of moving liquid–gas interfaces on detachment of colloidal particles from an air-dry solid surface. We hypothesized that hydrophobic colloids are more easily removed than hydrophilic colloids by a liquid–gas interface. We further hypothesized that more colloids detach from the solid surface when colloids are attached under unfavorable as compared to favorable conditions. We deposited hydrophilic and hydrophobic colloids under favorable and unfavorable conditions onto glass slides and quantified colloid detachment after passages of air–water interfaces as a function of number of passages and interfacial velocities. Experimental data were then compared with theoretical force calculations.
Section snippets
Colloids
We selected four different types of polystyrene colloids for the experiments: hydrophobic amidine-modified, hydrophilic amino-modified, hydrophilic carboxylate-modified, and hydrophobic sulfate-modified microspheres (Molecular Probes Inc., Eugene, OR). The carboxylate-modified and sulfate-modified microspheres were negatively charged while the amidine-modified and amino-modified microspheres were positively charged. All four colloids were fluorescent with an excitation wavelength of 505 nm and
DLVO forces
The DLVO profiles for the colloids and their interaction with the glass surface were calculated according to [20]: where is the electrostatic interaction energy, ϵ is the dielectric permittivity of the medium, R is the radius of the colloids, k is the Boltzmann constant, T is the absolute temperature; z is the ion valence, e is the electron charge, and are surface potential of the colloids and the glass slide,
Colloid removal during the passage of an air–water interface
Fig. 3 shows the images captured by confocal microscopy for the carboxylate-modified colloids before and after passages of the air–water interface for an interface velocity of 4 cm/h. The images represent typical patterns out of the 900 μm × 900 μm area of the 18 images taken. Only the images for carboxylate-modified colloids are presented here, the results for the other types of colloids were qualitatively similar. Image (a) represents the initial pattern of colloid deposition without passage of
Implications
In subsurface systems, like soils and sediments, moving air–water interfaces are common, e.g., during infiltration and drainage of water, air and water displace each other in continuous cycles. Such moving air–water interfaces have a profound effect on detachment of colloids from surfaces. As our experiments with polystyrene microspheres showed, colloids can be mobilized effectively when an air–water interface moves over an air-dried surface, suggesting that during infiltration into a dry soil,
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