Forced spreading of films and droplets of colloidal suspensions

2014 ◽  
Vol 742 ◽  
pp. 495-519 ◽  
Author(s):  
Leonardo Espín ◽  
Satish Kumar
Keyword(s):  
Particle Concentration ◽  
Colloidal Particles ◽  
Hard Spheres ◽  
Colloidal Suspension ◽  
Colloidal Suspensions ◽  
Concentration Gradients ◽  
Vertical Diffusion ◽  
Interface Evolution ◽  
Dependent Viscosity ◽  
Liquid Dynamics

AbstractWhen a thin film of a colloidal suspension flows over a substrate, uneven distribution of the suspended particles can lead to an uneven coating. Motivated by this phenomenon, we analyse the flow of perfectly wetting films and droplets of colloidal suspensions down an inclined plane. Lubrication theory and the rapid-vertical-diffusion approximation are used to derive a coupled pair of one-dimensional partial differential equations describing the evolution of the interface height and particle concentration. Precursor films are assumed to be present, the colloidal particles are taken to be hard spheres, and particle and liquid dynamics are coupled through a concentration- dependent viscosity and diffusivity. We find that for sufficiently high Péclet numbers, even small initial concentration inhomogeneities produce viscosity gradients that cause the film or droplet front to evolve continuously in time instead of travelling without changing shape as happens in the absence of colloidal particles. At high enough particle concentrations, particle diffusion can lead to the formation of long-lived secondary flow fronts in films. Our results suggest that particle concentration gradients can have a dramatic influence on interface evolution in flowing films and droplets, a finding which may be relevant for understanding the onset of patterns that are observed experimentally.

A macrotransport equation for the particle distribution in the flow of a concentrated, non-colloidal suspension through a circular tube

2013 ◽  
Vol 734 ◽  
pp. 219-252 ◽  
Author(s):  
Arun Ramachandran
Keyword(s):  
Cross Section ◽  
Colloidal Particles ◽  
Particle Distribution ◽  
Circular Tube ◽  
Colloidal Suspension ◽  
Classical Problem ◽  
Taylor Dispersion ◽  
Concentration Gradients ◽  
Concentrated Suspension ◽  
The Cross

AbstractA two-time-scale perturbation expansion is used to derive a cross-section-averaged convection–dispersion equation for the particle distribution in the flow of a concentrated suspension of neutrally buoyant, non-colloidal particles through a straight, circular tube. Since the cross-streamline motion of particles is governed by shear-induced migration, the Taylor-dispersion coefficient ${\mathscr{D}}_{eff} $ scales as ${U}^{\prime } {R}^{3} / {a}^{2} $, ${U}^{\prime } $, $R$ and $a$ being the characteristic velocity scale, the tube radius and the particle radius, respectively. Here ${\mathscr{D}}_{eff} $ is found to decrease monotonically with an increase in the particle concentration. The linear dependence of ${\mathscr{D}}_{eff} $ on ${U}^{\prime } $ implies that changes in the cross-section averaged axial concentration profile are dependent only on the total axial strain experienced by the suspension. This stipulates that the spatial evolution of a fluctuation in the concentration of particles in the flowing suspension, or the width of the mixing zone between two regions of different concentrations in the tube will be independent of the suspension velocity in the tube. A second interesting feature in particulate dispersion is that the effective velocity of the particulate phase is concentration-dependent, which, by itself (i.e. without considering Taylor dispersion), can produce either sharpening or relaxation of concentration gradients. In particular, shocks with positive concentration gradients along the flow direction can asymptotically evolve into time-independent distributions in an appropriately chosen frame of reference, and concentration pulses relax asymmetrically. These trends are contrasted with those expected from the classical problem of Taylor dispersion of a passive tracer in the same geometry. The results in this paper are especially relevant for suspension flows through microfluidic geometries, where the induction lengths for shear-induced migration are short.

Dielectrophoretic Mixing With Novel Electrode Geometry

2009 ◽  
Author(s):  
G. Naga Siva Kumar ◽  
Sushanta K. Mitra ◽  
Subir Bhattacharjee
Keyword(s):  
Electric Field ◽  
Cell Activation ◽  
Colloidal Particles ◽  
Colloidal Suspension ◽  
Three Dimensional ◽  
Colloidal Suspensions ◽  
Navier Stokes ◽  
Mixing Efficiency ◽  
Computational Domain ◽  
Element Analysis

Electrokinetic mixing of analytes at micro-scale is important in several biochemical applications like cell activation, DNA hybridization, protein folding, immunoassays and enzyme reactions. This paper deals with the modeling and numerical simulation of micromixing of two different types of colloidal suspensions based on principle of dielectrophoresis (DEP). A mathematical model is developed based on Laplace, Navier-Stokes, and convection-diffusion-migration equations to calculate electric field, velocity, and concentration distributions, respectively. Mixing of two colloidal suspensions is simulated in a three-dimensional computational domain using finite element analysis considering dielectrophoretic, gravitational and convective (advective)–diffusive forces. Phase shifted AC signal is applied to the alternating electrodes for achieving the mixing of two different colloidal suspensions. The results indicate that the electric field and DEP forces are maximum at the edges of the electrodes and become minimum elsewhere. As compared to curved edges, straight edges of electrodes have lower electric field and DEP forces. The results also indicate that DEP force decays exponentially along the height of the channel. The effect of DEP forces on the concentration profile is studied. It is observed that, the concentration of colloidal particles at the electrodes edges is very less compared to elsewhere. Mixing of two colloidal suspensions due to diffusion is observed at the interface of the two suspensions. The improvement in mixing after applying the repulsive DEP forces on the colloidal suspension is observed. Most of the mixing takes place across the slant edges of the triangular electrodes. The effect of electrode pairs and the mixing length on degree of mixing efficiency are also observed.

MODELING OF THE SURFACE TENSION OF COLLOIDAL SUSPENSIONS

2016 ◽  
Vol 24 (04) ◽  
pp. 1750050 ◽  
Author(s):  
ROGHAYEH HADIDIMASOULEH ◽  
MAZIAR SAHBA YAGHMAEE ◽  
REZA RIAHIFAR ◽  
BABAK RAISSI
Keyword(s):  
Surface Tension ◽  
Colloidal Particles ◽  
Colloidal Suspension ◽  
Colloidal Suspensions ◽  
Thermodynamic Relation ◽  
Fundamental Properties ◽  
Bubble Pressure ◽  
Air Water Interface ◽  
Numerous Experimental Data ◽  
Modified Equation

Surface tension is one of the fundamental properties of the colloids, which can be altered by concentration and size of colloidal particles. In the current work, modeling of the surface tension of suspension as it would be analyzed by maximum bubble pressure method has been performed. A new modified equation to correlate the surface tension with the bubble pressure is derived by applying fundamental thermodynamic relation considering the presence of particles in suspension and curvature of the interface between the particles and bubbles inside liquid. Moreover, the change of particles concentration in air–water interface due to capillary force is also considered. The predicted surface tension using the developed model has been verified by numerous experimental data with deviation less than 5% in most of cases. It was found that the calculated surface tension is altered by contact angle and particle radius as well as particle concentration. The obtained model may have potential application to predict the surface tension of colloidal suspension.

Mechanical Properties of Colloidal Gels

1989 ◽  
Vol 155 ◽  
Author(s):  
W.-H. Shih ◽  
J. Liu ◽  
W. Y. Shih ◽  
S. I. Kim ◽  
M. Sarikaya ◽  
...  
Keyword(s):  
Power Law ◽  
Particle Concentration ◽  
Colloidal Particles ◽  
Colloidal Suspension ◽  
The Other ◽  
Colloidal Gels ◽  
Yield Strain ◽  
Colloidal Gel ◽  
Type Of Behavior ◽  
Gel Network

A colloidal suspension can be either dispersed or flocculated depending on the interaction between the colloidal particles. If the interaction is repulsive, particles can relax to the minimum of the potential due to their neighboring particles, and the system can reach an equilibrium dispersed state. In the case of attractive interaction, particles form aggregates that settle to the bottom of the container. As the concentration of particles is increased, the overcrowding of the aggregates produces a continuous network throughout the suspension before they settle and a colloidal gel is formed. A major difference between a colloidal gel and a colloidal suspension is that the gel can sustain finite stress and is therefore viscoelastic. Previously we studied the storage modulus and the yield strain of boehmite gels and found that they are related to the particle concentration in a power-law fashion [1]. Similar scaling behavior of the shear modulus was found for other colloidal particulate networks by Buscall et al. [2]. We developed a scaling theory [1] which successfully explains the experimental results on boehmite gels. The theory further predicts that there can be two types of power-law behavior depending on the relative elastic strength of the clusters to that of the links between clusters within the gel network. Furthermore, there can be a crossover from one type of behavior to the other as the particle concentration is varied.

Colloidal assembly by ice templating

2016 ◽  
Vol 186 ◽  
pp. 61-76 ◽  
Author(s):  
Guruswamy Kumaraswamy ◽  
Bipul Biswas ◽  
Chandan Kumar Choudhury
Keyword(s):  
Size Distribution ◽  
Particle Concentration ◽  
Colloidal Particles ◽  
Cluster Formation ◽  
Polymer Chains ◽  
Concentration Gradients ◽  
Aqueous Dispersions ◽  
Colloidal Assembly ◽  
Colloidal Clusters ◽  
Ice Templating

We investigate ice templating of aqueous dispersions of polymer coated colloids and crosslinkers, at particle concentrations far below that required to form percolated monoliths. Freezing the aqueous dispersions forces the particles into close proximity to form clusters, that are held together as the polymer chains coating the particles are crosslinked. We observe that, with an increase in the particle concentration from about 106 to 108 particles per ml, there is a transition from isolated single particles to increasingly larger clusters. In this concentration range, most of the colloidal clusters formed are linear or sheet like particle aggregates. Remarkably, the cluster size distribution for clusters smaller than about 30 particles, as well as the size distribution of linear clusters, is only weakly dependent on the dispersion concentration in the range that we investigate. We demonstrate that the main features of cluster formation are captured by kinetic simulations that do not consider hydrodynamics or instabilities at the growing ice front due to particle concentration gradients. Thus, clustering of colloidal particles by ice templating dilute dispersions appears to be governed only by particle exclusion by the growing ice crystals that leads to their accumulation at ice crystal boundaries.

scholarly journals Hard spheres: crystallization and glass formation

2009 ◽  
Vol 367 (1909) ◽  
pp. 4993-5011 ◽  
Author(s):  
P. N. Pusey ◽  
E. Zaccarelli ◽  
C. Valeriani ◽  
E. Sanz ◽  
Wilson C. K. Poon ◽  
...  
Keyword(s):  
Glass Formation ◽  
Particle Concentration ◽  
Hard Spheres ◽  
Marked Change ◽  
Colloidal Suspensions ◽  
Crystal Nucleation ◽  
Free Energy Barrier ◽  
Coexistence Region ◽  
Main Effect ◽  
Dynamics Simulations

Motivated by old experiments on colloidal suspensions, we report molecular dynamics simulations of assemblies of hard spheres, addressing crystallization and glass formation. The simulations cover wide ranges of polydispersity s (standard deviation of the particle size distribution divided by its mean) and particle concentration. No crystallization is observed for s >0.07. For 0.02< s <0.07, we find that increasing the polydispersity at a given concentration slows down crystal nucleation. The main effect here is that polydispersity reduces the supersaturation since it tends to stabilize the fluid but to destabilize the crystal. At a given polydispersity (<0.07), we find three regimes of nucleation: standard nucleation and growth at concentrations in and slightly above the coexistence region; ‘spinodal nucleation’, where the free-energy barrier to nucleation appears to be negligible, at intermediate concentrations; and, at the highest concentrations, a new mechanism, still to be fully understood, which only requires small rearrangement of the particle positions. The cross-over between the second and third regimes occurs at a concentration, approximately 58 per cent by volume, where the colloid experiments show a marked change in the nature of the crystals formed and the particle dynamics indicate an ‘ideal’ glass transition.

scholarly journals Appearance of a Solitary Wave Particle Concentration in Nanofluids under a Light Field

Nanomaterials ◽  
2021 ◽  
Vol 11 (5) ◽  
pp. 1291
Author(s):  
Abram I. Livashvili ◽  
Victor V. Krishtop ◽  
Polina V. Vinogradova ◽  
Yuriy M. Karpets ◽  
Vyacheslav G. Efremenko ◽  
...  
Keyword(s):  
Solitary Wave ◽  
Wave Velocity ◽  
Light Field ◽  
Particle Concentration ◽  
Colloidal Suspension ◽  
Type Equation ◽  
Concentration Wave ◽  
Nanoparticle Concentration ◽  
Constant Intensity ◽  
Concentration Convection

In this study, the nonlinear dynamics of nanoparticle concentration in a colloidal suspension (nanofluid) were theoretically studied under the action of a light field with constant intensity by considering concentration convection. The heat and nanoparticle transfer processes that occur in this case are associated with the phenomenon of thermal diffusion, which is considered to be positive in our work. Two exact analytical solutions of a nonlinear Burgers-Huxley-type equation were derived and investigated, one of which was presented in the form of a solitary concentration wave. These solutions were derived considering the dependence of the coefficients of thermal conductivity, viscosity, and absorption of radiation on the nanoparticle concentration in the nanofluid. Furthermore, an expression was obtained for the solitary wave velocity, which depends on the absorption coefficient and intensity of the light wave. Numerical estimates of the concentration wave velocity for a specific nanofluid—water/silver—are given. The results of this study can be useful in the creation of next-generation solar collectors.

scholarly journals Shear thickening behavior in dense repulsive and attractive suspensions of hard spheres

Soft Matter ◽  
2021 ◽  
Author(s):  
Vikram Rathee ◽  
Alessandro Monti ◽  
Marco Edoardo Rosti ◽  
Amy Q Shen
Keyword(s):  
Flow Curve ◽  
Hard Spheres ◽  
Shear Thickening ◽  
Colloidal Suspensions ◽  
Thickening Behavior ◽  
The Stability

Shear thickening in stable dense colloidal suspensions is a reversible phenomenon and no hysteresis is observed in the flow curve measurements. However, a reduction in the stability of colloids promotes...

Stability of a Binary Colloidal Suspension and its effect on Colloidal Processing

1989 ◽  
Vol 155 ◽  
Author(s):  
Wan V. Shih ◽  
Wei-Heng Shih ◽  
Jun Liu ◽  
Ilhan A. Aksay
Keyword(s):  
Particle Size ◽  
Hard Sphere ◽  
Colloidal Particles ◽  
Colloidal Suspension ◽  
Fluid Phase ◽  
Ceramic Processing ◽  
Colloidal Processing ◽  
Interparticle Interactions ◽  
The Stability ◽  
Binary Suspension

The stability of a colloidal suspension plays an important role in colloidal processing of materials. The stability of the colloidal fluid phase is especially vital in achieving high green densities. By colloidal fluid phase, we refer to a phase in which colloidal particles are well separated and free to move about by Brownian motion, By controlling parameters such as pH, salt concentration, and surfactants, one can achieve high packing (green) densities in the repulsive regime where the suspension is well dispersed as a colloidal fluid, and low green densities in the attractive regime where the suspensions are flocculated [1,2]. While there is increasing interest in using bimodal suspensions to improve green densities, neither the stability of a binary suspension as a colloidal fluid nor the stability effects on the green densities have been studied in depth as yet. Traditionally, the effect of using bimodal-particle-size distribution has only been considered in terms of geometrical packing developed by Furnas and others [3,4]. This model is a simple packing concept and is used and useful for hard sphere-like repulsive interparticle interactions. With the advances in powder technology, smaller and smaller particles are available for ceramic processing. Thus, the traditional consideration of geometrial packing for the green densities of bimodal suspensions may not be enough. The interaction between particles must be taken into account.

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