Contact-line speed and morphology in vertical deposition of diluted colloids (original) (raw)
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Environmental Science & Technology, 2007
This study examines the deposition/release mechanisms involved in colloid retention under unfavorable conditions through theoretical analysis and laboratory column experiments. A Maxwell approach was utilized to estimate the coupled effects of primary-and secondary-minimum deposition. Theoretical analysis indicates that the secondary energy minimum plays a dominant role in colloid deposition even for nanosized particles (e.g., 20 nm) and primaryminimum deposition rarely happens for large colloids (e.g., 1000 nm) when diffusion is the dominant process. Polystyrene latex particles (30 and 1156 nm) and clean sand were used to conduct three-step column experiments at different solution ionic strengths, a constant pH of 10, and a flow rate of 0.0012 cm/s. Experimental results confirm that small colloids can also be deposited in secondary minima. Additional column experiments involving flow interruption further indicates that the colloids deposited in the secondary energy well can be spontaneously released to bulk solution when the secondary energy minimum is comparable to the average Brownian kinetic energy. Experimental collision efficiencies are in good agreement with Maxwell model predictions but different from the theoretical values calculated by the interfacial force boundary layer approximation. We propose a priori analytical approach to estimate collision efficiencies accounting for both primary-and secondary-minimum deposition and suggest that the reversibility of colloid (e.g., viruses and bacteria) deposition must be considered in transport models for accurate predictions of their travel time in the subsurface environments.
Interaction of bi-dispersed particles with contact line in an evaporating colloidal drop
The deposition behavior of an inkjet-printed aqueous colloidal mixture of micro-and nanoparticles onto a glass substrate with systematically varied wettability has been investigated using fluorescence microscopy. Real-time bottom-view images show that particles inside an evaporating drop rearrange themselves near the drop contact line according to their sizes, where smaller particles tend to deposit closer to the contact line compared to the larger ones. By increasing substrate wettability, particles in the bi-dispersed mixture can be further separated compared to those on substrates of poor wettability. This is primarily because, during different stages of evaporation, the interplay of surface tension, drag due to evaporative flow, and particle–substrate interactions rearrange particles inside a colloidal drop near the contact line region. Forces acting on particles determine the extent to which particles enhance contact line pinning, which ultimately determines the final deposition morphology of particles from a bi-dispersed colloidal mixture.
Moving Contact Lines of a Colloidal Suspension in the Presence of Drying
Langmuir, 2006
This article presents the first experimental study of an advancing contact line for a colloidal suspension. A competition between the hydrodynamic flow due to the drop velocity and the drying is exhibited: drying accounts for particle agglomeration that pins the contact line whereas the liquid flow dilutes the agglomerated particles and allows the contact line to advance continuously. The dilution dominates at low concentration and high velocity, but at high concentration and low velocity, the contact line can be pinned by the particle agglomeration, which leads to a stick-slip motion of the contact line. The calculation of the critical speed splitting both regimes gives an order of magnitude comparable to that of experiments. Moreover, a model of agglomeration gives an estimation of both the size of the wrinkles formed during stick-slip and the force exerted by the wrinkle on the contact line.
Spontaneous Pattern Formation by Dip Coating of Colloidal Suspensions on Homogeneous Surfaces
Langmuir, 2007
We study the slow withdrawal of a partially wet vertical plate at velocity U from a suspension of well-wet particles. Periodic horizontal striped assemblies form spontaneously at the three-phase contact line on energetically uniform surfaces. Stripe width and spacing depend on the withdrawal velocity U relative to a transition velocity U t. Thick stripes separated by large spaces form for U < U t. For U > U t , thin stripes separated by small spaces form. The stripe spacing is reduced by an order of magnitude and varies weakly with U until a maximum velocity is reached at which the stripes fail to form. A partially wet surface can entrain a meniscus. For U < U t , the meniscus forms a finite contact angle wedge with a pinned contact line. As the plate moves upward, it stretches the meniscus until it becomes too heavy to be retained by the wet, porous network provided by the particles at the contact line. The contact line then jumps backward to find a new equilibrium location, and the process begins anew. For U > U t , we infer that a film of thickness h is entrained above the meniscus. When h is smaller than the particle diameter D, particles aggregate where the entrained film thickens to match up to the wetting meniscus. When an entrained particle becomes exposed to air by evaporation, it becomes the new pinning site from which the next film is entrained. The film thickness h increases with U; at some velocity, h becomes comparable to D. Particles flow into the film and deposit there in a disordered manner. A diagram summarizing particle deposition is developed as a function of D, U, and h.
On the parameters influencing the deposition of polystyrene colloidal crystals
Materials Science and Engineering: C, 2008
Colloidal crystals of polystyrene particles of 1.0, 1.4 and 2.8 μm diameter have been prepared by vertical deposition. The influence of parameters such as temperature, particle size and concentration as well as dispersion medium has been studied. The size of domain and the crystalline structure of the particle arrays have been analyzed by optical microscopy. The quality of the crystals has been improved (minimizing cracks) by controlling sedimentation (density matching), evaporation (volatility of the medium) and drying (co-solvents).
JOURNAL OF CHEMICAL ENGINEERING OF JAPAN, 2022
Branched occulants are more e cient at improving occulation rates than linear occulants, but the reason is not well understood. The adsorption of linear polymers is known to increase the hydrodynamic radius of particles for e ective collisions between particles and subsequently improve occulation rate. To reveal such an e ect with branched polymers, we measured the thickness of the adsorbed polyelectrolyte layer and their electrophoretic mobility as a function of time for polyelectrolytes with di erent degrees of branching. The results show that all polyelectrolytes adsorbed on the particles undergo a relaxation e ect, where the thickness of the adsorbed layer continuously decreases as a function of time. The branched polymers exhibited a thick layer regardless of ionic strength due to branching points that disturbed smooth reconformation when adsorbed to the particles. However, linear polymers consistently showed a thinner layer thickness than branched polymers because of their exible structure, which was prone to relaxation e ects. The most signi cant layer thickness was observed at low ionic strength, and the electrophoretic mobility con rmed that adsorption was kinetically controlled. The thick adsorbed layer of branched polymers explains the commonly observed trend, where branched polymers show better occulating abilities than linear polymers.
Shape Dependent Colloidal Deposition and Detachment
Advanced Theory and Simulations
Models of deposition and detachment dynamics of different shaped anisotropic colloids are reported to understand how equilibrium deposited amounts compare to spherical colloids. For different shaped colloids including spheres, ellipsoids, toroids, and buckled particles with varying aspect ratios, interaction potentials with substrates are computed using the Derjaguin approximation. Using these potentials, the Smoluchowski equation is used to model the dynamics of deposition and detachment versus particle-substrate attraction and aspect ratio for each particle shape. Average times for deposition and detachment and their ratio show steady-state deposited amounts can be enhanced by several orders of magnitude for different particle shapes compared to spherical colloids of the same volume. From a mechanistic standpoint, the present findings indicate how local Gaussian curvature of different particle shapes can lead to stronger adhesive interactions, longer detachment times, and higher deposited amounts compared to spherical colloids, which provides general design rules for controlling and optimizing colloidal deposition.
Simulation of surface deposition of colloidal spheres under flow
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004
The deposition kinetics of stable colloidal spheres from dilute dispersions flowing through a model pore in the high surface coverage range has been investigated. After deposition of each particle, the new flow field is calculated by using a flow simulator that solves the Navier-Stokes equations. The results provide both the surface coverage Γ and the hydrodynamic thickness δ h of deposited layer as a function of a "hydrodynamic shadowing" Péclet number Pe hs , which has been defined to be relevant for this process. A first result is that for Pe hs ≤ 1, both the surface coverage Γ and the equivalent hydrodynamic thickness of deposited layer show a definite plateau. In this regime, the plateau value of Γ at high surface coverage as well as deposition kinetics are identical to those given by the random sequential adsorption theory (RSA). A second result is that for Pe hs ≥ 1 both Γ and δ h are decreasing function of flow strength. In addition, although limited simulations have been carried out in that regime, the surface coverage seem to decrease as Pe −1/3 hs , in agreement with both a theoretical approach based on diffusion layer approximation and previous experimental results. This good agreement suggests that the numerical simulation approach described in this paper is valid so that, it could be used to investigate multi-layer deposition and the corresponding porous media permeability damage by introducing an attractive particle-particle potential.
Colloidal Sedimentation in Polymer Solutions
Macromolecules, 1998
We report sedimentation measurements of small colloidal particles through a nonadsorbing polymer solution. The particle sedimentation velocity vc(φc,Cp), as a function of the colloid volume fraction φc and the polymer concentration Cp, is found to be affected by both the microscopic viscosity ηc(Cp) experienced by the particles in the solution and the polymer-induced depletion interaction between the particles. The measurements reveal a large effect of the depletion interaction on colloidal sedimentation in the high-φc samples. The experiment demonstrates the effectiveness of using the sedimentation method to study interaction-related phenomena in colloidal suspensions.