Particle deposition

Last updated November 30, 2021

Particle deposition is the spontaneous attachment of particles to surfaces. The particles in question are normally colloidal particles, while the surfaces involved may be planar, curved, or may represent particles much larger in size than the depositing ones (e.g., sand grains). Deposition processes may be triggered by appropriate hydrodynamic flow conditions and favorable particle-surface interactions. Depositing particles may just form a monolayer which further inhibits additional particle deposition, and thereby one refers to surface blocking. Initially attached particles may also serve as seeds for further particle deposition, which leads to the formation of thicker particle deposits, and this process is termed as surface ripening or fouling. While deposition processes are normally irreversible, initially deposited particles may also detach. The latter process is known as particle release and is often triggered by the addition of appropriate chemicals or a modification in flow conditions.

Microorganisms may deposit to surfaces in a similar fashion as colloidal particles. When macromolecules, such as proteins, polymers or polyelectrolytes attach to surfaces, one rather calls this process adsorption. While adsorption of macromolecules largely resembles particle deposition, macromolecules may substantially deform during adsorption. The present article mainly deals with particle deposition from liquids, but similar process occurs when aerosols or dust deposit from the gas phase.

Initial stages

A particle may diffuse to a surface in quiescent conditions, but this process is inefficient as a thick depletion layer develops, which leads to a progressive slowing down of the deposition. When particle deposition is efficient, it proceeds almost exclusively in a system under flow. In such conditions, the hydrodynamic flow will transport the particles close to the surface. Once a particle is situated close to the surface, it will attach spontaneously, when the particle-surface interactions are attractive. In this situation, one refers to favorable deposition conditions. When the interaction is repulsive at larger distances, but attractive at shorter distances, deposition will still occur but it will be slowed down. One refers to unfavorable deposition conditions here. The initial stages of the deposition process can be described with the rate equation^[1]

{d\Gamma  \over dt}=kc

where $\Gamma$ ; is the number density of deposited particles, $t$ is the time, $c$ the particle number concentration, and $k$ the deposition rate coefficient. The rate coefficient depends on the flow velocity, flow geometry, and the interaction potential of the depositing particle with the substrate. In many situations, this potential can be approximated by a superposition of attractive van der Waals forces and repulsive electrical double layer forces and can be described by DLVO theory. When the charge of the particles is of the same sign as the substrate, deposition will be favorable at high salt levels, while it will be unfavorable at lower salt levels. When the charge of the particles is of the opposite sign as the substrate, deposition is favorable for all salt levels, and one observes a small enhancement of the deposition rate with decreasing salt level due to attractive electrostatic double layer forces. Initial stages of the deposition process are relatively similar to the early stages of particle heteroaggregation, whereby one of the particles is much larger than the other.

Blocking

When depositing particles repel each other, the deposition will stop by the time when enough particles have deposited. At one point, such a surface layer will repel any particles that may still make attempts to deposit. The surface is said to be saturated or blocked by the deposited particles. The blocking process can be described by the following equation^[2]

{d\Gamma  \over dt}=kcB(\Gamma )

where $B(\Gamma )$ is the surface blocking function. When there are no deposited particles, $\Gamma =0$ and $B(0)=1$ . With increasing number density of deposited particles, the blocking function decreases. The surface saturates at $\Gamma =\Gamma _{0}$ and $B(\Gamma _{0})=0$ . The simplest blocking function is^[3]

B(\Gamma )=1-\Gamma /\Gamma _{0}

and it is referred to as the Langmuir blocking function, as it is related to the Langmuir isotherm.

Jamming in the random sequential adsorption (RSA) of circular disks. Random Sequential Adsorption Disks1.png — Jamming in the random sequential adsorption (RSA) of circular disks.

The blocking process has been studied in detail in terms of the random sequential adsorption (RSA) model.^[4] The simplest RSA model related to deposition of spherical particles considers irreversible adsorption of circular disks. One disk after another is placed randomly at a surface. Once a disk is placed, it sticks at the same spot, and cannot be removed. When an attempt to deposit a disk would result in an overlap with an already deposited disk, this attempt is rejected. Within this model, the surface is initially filled rapidly, but the more one approaches saturation the slower the surface is being filled. Within the RSA model, saturation is referred to as jamming. For circular disks, jamming occurs at a coverage of 0.547. When the depositing particles are polydisperse, much higher surface coverage can be reached, since the small particles will be able to deposit into the holes in between the larger deposited particles. On the other hand, rod like particles may lead to much smaller coverage, since a few misaligned rods may block a large portion of the surface.

Since the repulsion between particles in aqueous suspensions originates from electric double layer forces, the presence of salt has an important effect on surface blocking. For small particles and low salt, the diffuse layer will extend far beyond the particle, and thus create an exclusion zone around it. Therefore, the surface will be blocked at a much lower coverage than what would be expected based on the RSA model.^[5] At higher salt and for larger particles, this effect is less important, and the deposition can be well described by the RSA model.

Ripening

When the depositing particles attract each other, they will deposit and aggregate at the same time. This situation will result in a porous layer made of particle aggregates at the surface, and is referred to as ripening. The porosity of this layer will depend whether the particle aggregation process is fast or slow. Slow aggregation will lead to a more compact layer, while fast aggregation to a more porous one. The structure of the layer will resemble the structure of the aggregates formed in the later stages of the aggregation process.

Experimental techniques

Particle deposition can be followed by various experimental techniques. Direct observation of deposited particles is possible with an optical microscope, scanning electron microscope, or the atomic force microscope. Optical microscopy has the advantage that the deposition of particles can be followed in real time by video techniques and the sequence of images can be analyzed quantitatively.^[6] On the other hand, the resolution of optical microscopy requires that the particle size investigated exceeds at least 100 nm.

An alternative is to use surface sensitive techniques to follow particle deposition, such as reflectivity, ellipsometry, surface plasmon resonance, or quartz crystal microbalance.^[5] These techniques can provide information on the amount of particles deposited as a function of time with good accuracy, but they do not permit to obtain information concerning the lateral arrangement of the particles.

Another approach to study particle deposition is to investigate their transport in a chromatographic column. The column is packed with large particles or with a porous medium to be investigated. Subsequently, the column is flushed with the solvent to be investigated, and the suspension of the small particles is injected at the column inlet. The particles are detected at the outlet with a standard chromatographic detector. When particles deposit in the porous medium, they will not arrive at the outlet, and from the observed difference the deposition rate coefficient can be inferred.

Relevance

Particle deposition occurs in numerous natural and industrial systems. Few examples are given below.

Coatings and surface functionalization. Paints and adhesives often are concentrated suspensions of colloidal particles, and in order to adhere well to the surface the particles must deposit to the surface in question. Deposits of a monolayer of colloidal particles can be used to pattern the surface on a μm or nm scale, a process referred to as colloidal lithography.^[7]
Filters and filtration membranes. When particle deposit to filters or filtration membranes, they lead to pore clogging a membrane fouling.^[8] When designing well functioning membranes, particle deposition must be avoided, and proper functionalization of the membranes is essential.
Deposition of microorganisms . Microorganisms may deposit similarly to colloidal particles. This deposition is a desired phenomenon in subsurface waters, as the aquifer filters out eventually injected microorganisms during the recharge of aquifers.^[9] On the other hand, such deposition is highly undesired at the surface of human teeth as it represent the origin of dental plaques. Deposition of microorganisms is also relevant in the formation of biofilms.

Related Research Articles

A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. However, some definitions specify that the particles must be dispersed in a liquid, and others extend the definition to include substances like aerosols and gels. The term colloidal suspension refers unambiguously to the overall mixture. A colloid has a dispersed phase and a continuous phase. The dispersed phase particles have a diameter of approximately 1 nanometre to 1 micrometre.

Electrophoresis is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field. Electrophoresis of positively charged particles (cations) is sometimes called cataphoresis, while electrophoresis of negatively charged particles (anions) is sometimes called anaphoresis.

Ultrafiltration (UF) is a variety of membrane filtration in which forces like pressure or concentration gradients lead to a separation through a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained in the so-called retentate, while water and low molecular weight solutes pass through the membrane in the permeate (filtrate). This separation process is used in industry and research for purifying and concentrating macromolecular (10³–10⁶ Da) solutions, especially protein solutions.

Zeta potential is the electrical potential at the slipping plane. This plane is the interface which separates mobile fluid from fluid that remains attached to the surface.

The DLVO theory explains the aggregation of aqueous dispersions quantitatively and describes the force between charged surfaces interacting through a liquid medium. It combines the effects of the van der Waals attraction and the electrostatic repulsion due to the so-called double layer of counterions. The electrostatic part of the DLVO interaction is computed in the mean field approximation in the limit of low surface potentials - that is when the potential energy of an elementary charge on the surface is much smaller than the thermal energy scale, $. For two spheres of radius each having a charge separated by a center-to-center distance in a fluid of dielectric constant containing a concentration of monovalent ions, the electrostatic potential takes the form of a screened-Coulomb or Yukawa potential,$

An artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial processes since the middle of twentieth century. A wide variety of synthetic membranes is known. They can be produced from organic materials such as polymers and liquids, as well as inorganic materials. The most of commercially utilized synthetic membranes in separation industry are made of polymeric structures. They can be classified based on their surface chemistry, bulk structure, morphology, and production method. The chemical and physical properties of synthetic membranes and separated particles as well as a choice of driving force define a particular membrane separation process. The most commonly used driving forces of a membrane process in industry are pressure and concentration gradients. The respective membrane process is therefore known as filtration. Synthetic membranes utilized in a separation process can be of different geometry and of respective flow configuration. They can also be categorized based on their application and separation regime. The best known synthetic membrane separation processes include water purification, reverse osmosis, dehydrogenation of natural gas, removal of cell particles by microfiltration and ultrafiltration, removal of microorganisms from dairy products, and Dialysis.

Surface charge is a two-dimensional surface with non-zero electric charge. These electric charges are constrained on this 2-D surface, and surface charge density, measured in coulombs per square meter (C•m⁻²), is used to describe the charge distribution on the surface. The electric potential is continuous across a surface charge and the electric field is discontinuous, but not infinite; this is unless the surface charge consists of a dipole layer. In comparison, the potential and electric field both diverge at any point charge or linear charge.

Fouling Accumulation of unwanted material on solid surfaces

Fouling is the accumulation of unwanted material on solid surfaces. The fouling materials can consist of either living organisms (biofouling) or a non-living substance. Fouling is usually distinguished from other surface-growth phenomena in that it occurs on a surface of a component, system, or plant performing a defined and useful function and that the fouling process impedes or interferes with this function.

Particle agglomeration refers to formation of assemblages in a suspension and represents a mechanism leading to the functional destabilization of colloidal systems. During this process, particles dispersed in the liquid phase stick to each other, and spontaneously form irregular particle assemblages, flocs, or agglomerates. This phenomenon is also referred to as coagulation or flocculation and such a suspension is also called unstable. Particle agglomeration can be induced by adding salts or other chemicals referred to as coagulant or flocculant.

The Gibbs adsorption isotherm for multicomponent systems is an equation used to relate the changes in concentration of a component in contact with a surface with changes in the surface tension, which results in a corresponding change in surface energy. For a binary system, the Gibbs adsorption equation in terms of surface excess is:

A double layer is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object. The first layer, the surface charge, consists of ions adsorbed onto the object due to chemical interactions. The second layer is composed of ions attracted to the surface charge via the Coulomb force, electrically screening the first layer. This second layer is loosely associated with the object. It is made of free ions that move in the fluid under the influence of electric attraction and thermal motion rather than being firmly anchored. It is thus called the "diffuse layer".

A Pickering emulsion is an emulsion that is stabilized by solid particles which adsorb onto the interface between the two phases. This type of emulsion was named after S.U. Pickering, who described the phenomenon in 1907, although the effect was first recognized by Walter Ramsden in 1903.

Peptization or deflocculation is the process of converting precipitate into colloid by shaking it with a suitable electrolyte called peptizing agent.

Bacterial adhesion involves the attachment of bacteria on the surface. This interaction plays an important role in natural system as well as in environmental engineering. The attachment of biomass on the membrane surface will result in membrane fouling, which can significantly reduce the efficiency of the treatment system using membrane filtration process in wastewater treatment plants. The low adhesion of bacteria to soil is essential key for the success of in-situ bioremediation in groundwater treatment. However, the contamination of pathogens in drinking water could be linked to the transportation of microorganisms in groundwater and other water sources. Controlling and preventing the adverse impact of the bacterial deposition on the aquatic environment need a deeply understanding about the mechanisms of this process. DLVO theory has been used extensively to describe the deposition of bacteria in many current researches.

Mucoadhesion describes the attractive forces between a biological material and mucus or mucous membrane. Mucous membranes adhere to epithelial surfaces such as the gastrointestinal tract (GI-tract), the vagina, the lung, the eye, etc. They are generally hydrophilic as they contain many hydrogen macromolecules due to the large amount of water within its composition. However, mucin also contains glycoproteins that enable the formation of a gel-like substance. Understanding the hydrophilic bonding and adhesion mechanisms of mucus to biological material is of utmost importance in order to produce the most efficient applications. For example, in drug delivery systems, the mucus layer must be penetrated in order to effectively transport micro- or nanosized drug particles into the body. Bioadhesion is the mechanism by which two biological materials are held together by interfacial forces.

A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ; nuclear membranes, which cover a cell nucleus; and tissue membranes, such as mucosae and serosae. Synthetic membranes are made by humans for use in laboratories and industry.

Adsorption of polyelectrolytes on solid substrates is a surface phenomenon where long-chained polymer molecules with charged groups bind to a surface that is charged in the opposite polarity. On the molecular level, the polymers do not actually bond to the surface, but tend to "stick" to the surface via intermolecular forces and the charges created by the dissociation of various side groups of the polymer. Because the polymer molecules are so long, they have a large amount of surface area with which to contact the surface and thus do not desorb as small molecules are likely to do. This means that adsorbed layers of polyelectrolytes form a very durable coating. Due to this important characteristic of polyelectrolyte layers they are used extensively in industry as flocculants, for solubilization, as supersorbers, antistatic agents, as oil recovery aids, as gelling aids in nutrition, additives in concrete, or for blood compatibility enhancement to name a few.

Protein adsorption refers to the adhesion of proteins to solid surfaces. This phenomenon is an important issue in the food processing industry, particularly in milk processing and wine and beer making. Excessive adsorption, or protein fouling, can lead to health and sanitation issues, as the adsorbed protein is very difficult to clean and can harbor bacteria, as is the case in biofilms. Product quality can be adversely affected if the adsorbed material interferes with processing steps, like pasteurization. However, in some cases protein adsorption is used to improve food quality, as is the case in fining of wines.

Double layer forces occur between charged objects across liquids, typically water. This force acts over distances that are comparable to the Debye length, which is on the order of one to a few tenths of nanometers. The strength of these forces increases with the magnitude of the surface charge density. For two similarly charged objects, this force is repulsive and decays exponentially at larger distances, see figure. For unequally charged objects and eventually at shorted distances, these forces may also be attractive. The theory due to Derjaguin, Landau, Verwey, and Overbeek (DLVO) combines such double layer forces together with Van der Waals forces in order to estimate the actual interaction potential between colloidal particles.

A depletion force is an effective attractive force that arises between large colloidal particles that are suspended in a dilute solution of depletants, which are smaller solutes that are preferentially excluded from the vicinity of the large particles. One of the earliest reports of depletion forces that lead to particle coagulation is that of Bondy, who observed the separation or 'creaming' of rubber latex upon addition of polymer depletant molecules to solution. More generally, depletants can include polymers, micelles, osmolytes, ink, mud, or paint dispersed in a continuous phase.

References

↑ W. B. Russel, D. A. Saville, W. R. Schowalter,Colloidal Dispersions,Cambridge University Press, 1989.
↑ M. Elimelech, J. Gregory, X. Jia, R. Williams, Particle Deposition and Aggregation: Measurement, Modelling and Simulation, Butterworth-Heinemann, 1998.
↑ Z. Adamczyk, Adv. Colloid Interface Sci. 2003, 100, 267-347.
↑ J. W. Evans, Rev. Mod. Phys. 65 (1993) 1281-1329.
1 2 M. R. Bohmer, E. A. van der Zeeuw, G. J. M. Koper, J. Colloid Interface Sci. 197 (1998) 242-250.
↑ Y. Luthi, J. Ricka, J. Colloid Interface Sci. 206 (1998) 302-313.
↑ R. Michel, I. Reviakine, D. S. Sutherland, G. Fokas, G. Csucs, G. Danuser, N. D. Spencer, M. Textor, Langmuir 18 (2002) 8580-8586.
↑ X. Zhu, M. Elimelech, Environ. Sci. Technol. 31 (1997) 3654-3662.
↑ S. F. Simoni, H. Harms, T. N. P. Bosma, A. J. B. Zehnder, Environ. Sci. Technol. 32 (1998) 2100-2105

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[russel-1] W. B. Russel, D. A. Saville, W. R. Schowalter,Colloidal Dispersions,Cambridge University Press, 1989.

[gregory-2] M. Elimelech, J. Gregory, X. Jia, R. Williams, Particle Deposition and Aggregation: Measurement, Modelling and Simulation, Butterworth-Heinemann, 1998.

[adamczyk-3] Z. Adamczyk, Adv. Colloid Interface Sci. 2003, 100, 267-347.

[evans-4] J. W. Evans, Rev. Mod. Phys. 65 (1993) 1281-1329.

[bohmer-5] 1 2 M. R. Bohmer, E. A. van der Zeeuw, G. J. M. Koper, J. Colloid Interface Sci. 197 (1998) 242-250.

[6] Y. Luthi, J. Ricka, J. Colloid Interface Sci. 206 (1998) 302-313.

[7] R. Michel, I. Reviakine, D. S. Sutherland, G. Fokas, G. Csucs, G. Danuser, N. D. Spencer, M. Textor, Langmuir 18 (2002) 8580-8586.

[zhu-8] X. Zhu, M. Elimelech, Environ. Sci. Technol. 31 (1997) 3654-3662.

[9] S. F. Simoni, H. Harms, T. N. P. Bosma, A. J. B. Zehnder, Environ. Sci. Technol. 32 (1998) 2100-2105

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]