Difference between revisions of "Structure of adhesive emulsions"

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(New page: J. Bibette,tJ T. G. Mason,$ Hu Gang,$ D. A. Weitz,*J and P. Poulint "Structure of adhesive emulsions" Entry by Fei Pu, AP 225, Fall 2012 '''Keywords:''' contact angle, [[surfac...)
 
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J. Bibette,tJ T. G. Mason,$ Hu Gang,$ D. A. Weitz,*J and P. Poulint
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J. Bibette,J T. G. Mason,Hu Gang,$ D. A. Weitz,J and P. Poulint
  
 
"Structure of adhesive emulsions"
 
"Structure of adhesive emulsions"
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==Summary==
 
==Summary==
 +
Emulsions are inherently unstable dispersions of one, normally immiscible, fluid in a second.' Nevertheless, with appropriate surfactant molecules, or other surface active species, emulsions can be made to be stable nearly indefinitely.
  
 
When oil is dropped in water emulsions, the interactions between the droplets are so strong that they adhere together and retain their integral shapes. The structure of the strongly adhesive emulsions reflects a complex interplay among the strength of the adhesion, the droplet volume fraction, contact angle phi, and the time evolution of the adhesion. Initially, strong adhesion of the droplets leads to the formation of an emulsion gel. Moreover, the gel possesses a well-defined characteristic length scale, d,, as evidenced by an intense ring of small angle light scattering. The characteristic length scale decreases as the droplet volume fraction increases. At low phi, the gelation mechanism is controlled by diffusion-limited cluster
 
When oil is dropped in water emulsions, the interactions between the droplets are so strong that they adhere together and retain their integral shapes. The structure of the strongly adhesive emulsions reflects a complex interplay among the strength of the adhesion, the droplet volume fraction, contact angle phi, and the time evolution of the adhesion. Initially, strong adhesion of the droplets leads to the formation of an emulsion gel. Moreover, the gel possesses a well-defined characteristic length scale, d,, as evidenced by an intense ring of small angle light scattering. The characteristic length scale decreases as the droplet volume fraction increases. At low phi, the gelation mechanism is controlled by diffusion-limited cluster
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==Materials and Methods==
 
==Materials and Methods==
  
Colloidal particles consisting of one smooth and one rough sphere were synthesized following a modified synthesis by Kim et al. (27). Roughness on the seed particles was obtained through adsorption of polystyrene particles nucleated during polymerization. The synthesized colloids were washed and redispersed in 0.3% w∕w aqueous polyvinyl alcohol (Mw ¼ 30–50 kg∕mol).
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The crude emulsion is formed from silicon oil droplets in water, stabilized by sodium dodecyl sulfate (SDS). Highly monodisperse emulsion droplets, about 0.6 pm in diameter, are obtained by repeated crystallization fractionation. The surfactant concentration in the continuous phase is adjusted to about 0.01 M, slightly above the critical micelle concentration, ensuring the stability of the emulsion. To induce adhesion between the droplets, sodium chloride is added to the continuous phase.
  
Monte Carlo simulations were used in the canonical ensemble (NVT) to calculate the probability distribution of the cluster. Also, The free energy of clusters of different sizes was calculated using grand-canonical Monte Carlo (GCMC) simulations on single clusters (41).
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==Results==
  
==Results & Discussion==
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The strength of the adhesion is strongly temperature dependent: Above an onset temperature, T, the droplets remain repulsive, whereas adhesion is induced between the droplets when the temperature is reduced below T, and the strength of the adhesion increases as the temperature is further decreased. We believe that this may be associated with the formation of a nonzero
 +
contact angle, phi, between droplets, as has been observed for larger, polydisperse emulsion droplets. The onset temperature
 +
depends sensitively on the concentration of sodium chloride, increasing with increasing NaCl concentration. All experiments are carried out by rapidly lowering the temperature several degrees below T, to induce the attraction, ensuring strong adhesion between
 +
the droplets. The structure of the flocculating emulsions are directly observed using a phase contrast microscope with a variable temperature stage.
  
When two colloids overlap each other, the depletion entropy increases, and such phenomenon makes the colloids attract more closely. The effect of rough particles interacting with smooth particles and other rough particles was recorded and analyzed. At higher concentrations and thus stronger attractions, the roughness anisotropic colloidal particles spontaneously organized into clusters, in which the attractive parts constitute the core of the aggregate and the non-attractive rough sides are located at the outside. These structures look like micelles, as shown below in Figure 1.  
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While the temperature of the emulsion is maintained above T,, the droplets undergo random Brownian motion, and there is a sufficiently large, short-range repulsive barrier between them to ensure stability against flocculation. When the temperature is lowered several degrees below T, a strong adhesion develops between the droplets. However, while the droplets adhere to one another, they retain their integrity and do not coalesce.
  
[[Image:Aggregate.jpg|thumb|center|400px|
 
Figure 1. The rough and smooth colloidal particles interact and spontaneously bond into clusters, which could be called [[colloidal micelles]]]]
 
  
 +
The gel also depends on the contact angle phi. At phi = 0.005, the structure of the gel at short length scales consists of a highly random, tenuous network of droplets. The distinct branches of the network are roughly a single droplet in thickness. As phi is increased to 0.01 and then to 0.05, the average thickness of the branches increases slightly, but the structure still exhibits a random, tenuous network at short length scales.  As phi is increased still further, the average thickness of the branches increases substantially and the tenuous network is no longer observed at short length scales. Instead, the gel has a dramatically different
 +
appearance, with a random, weak and free-flowing network.
  
Monte Carlo simulation was further done on the smooth and rough colloids, shown in Figure 2. As time went on, it seems like smooth particles attracted into clusters and formed the core of the micelles and that the rough particles stayed outside and surrounded the core.
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==Discussion==
  
[[Image:Colloid Micelle.jpg|thumb|center|400px|
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A remarkable feature of these emulsion gels is the fact that they form a tenuous network, which must maintain some inherent rigidity, despite the fact that it is composed entirely of liquid droplets. This rigidity has a profound implication about the nature of the adhesion: It suggests that there must be no slip between two adjacent droplets. If slip could occur, the droplets would be able to rotate about their bonds, and the resultant rearrangement would lead to a significantly less tenuous structure. This suggests that a rigid film is formed by the adhesion between neighboring droplets. One possibility is that the surfactant forms a solid layer at the interface, which prevents re-arrangement of the atoms on the surface.  
Figure 2. Smooth particles clustered on the inside of the micelles while the rough particles surrounded the outside.]]
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Finally,cluster size distributions changed as interactions increased and geometry overlapped more. From Figure 3, it's clear that as density, p, increased, clusters were more prone and easily formed. When density was low at the beginning, the particles flowed freely and even repelled. Only when density reached a threshold that the surface energy and forces favored clustering.
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[[Image:Cluster size distributions.jpg|thumb|center|400px|
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Figure 3. As density of particles increased, they overlapped more and formed clusters.]]
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==Soft Matter Applications==
 
==Soft Matter Applications==
  
Due to the available variety of colloids and their straightforward assembly even between different patch sizes, it is expected that
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Under favorable conditions emulsification offers the opportunity to incorporate almost any ratio of one fluid in another. This leads to a wide range of important technological applications, ranging from cosmetics to coatings and from foods to medicines.
these soft colloidal particles with smooth and rough surfaces could self-assemble in a controlled manner into superstructures with desired topology and properties. This has significant applications. For example, the virus macromolecules, protein subunits, and building cell blocks in our body are often complex and challenging to identify key elements for self-assembly processes. By mimicking such self-assembly processes on a colloidal scale, insights into the paramount elements that control the assembly can be obtained in situ and applied to build up superstructures with new and desirable properties. The findings in this article have fundamental and practical importance in the field of colloidal and macromolecular self assembly.
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Emulsions are also fascinating dispersions of deformable spheres, which exhibit novel colloidal properties. For example, the interaction between droplets can be made strongly adhesive, ultimately leading to a nonzero contact angle between droplets. If the droplets retain their integrity, this adhesion causes flocculation of the emulsion droplets. Emulsion flocculation, or creaming, is a commonly encountered phenomenon. If the phenomenon is understood, it could be used in many different colloidal applications.

Revision as of 19:50, 13 October 2012

J. Bibette,J T. G. Mason,Hu Gang,$ D. A. Weitz,J and P. Poulint

"Structure of adhesive emulsions"

Entry by Fei Pu, AP 225, Fall 2012


Keywords: contact angle, surface forces, adhesives, droplets


Summary

Emulsions are inherently unstable dispersions of one, normally immiscible, fluid in a second.' Nevertheless, with appropriate surfactant molecules, or other surface active species, emulsions can be made to be stable nearly indefinitely.

When oil is dropped in water emulsions, the interactions between the droplets are so strong that they adhere together and retain their integral shapes. The structure of the strongly adhesive emulsions reflects a complex interplay among the strength of the adhesion, the droplet volume fraction, contact angle phi, and the time evolution of the adhesion. Initially, strong adhesion of the droplets leads to the formation of an emulsion gel. Moreover, the gel possesses a well-defined characteristic length scale, d,, as evidenced by an intense ring of small angle light scattering. The characteristic length scale decreases as the droplet volume fraction increases. At low phi, the gelation mechanism is controlled by diffusion-limited cluster aggregation. However, at higher phi, the short range structure is more compact, rather than fractal, and a different mechanism must be responsible for the gelation. If the strength of the adhesion is increased still further, the droplets become more deformed, resulting in massive restructuring of the emulsion gel. The structure fractures into independent, more compact flocs, eliminating the overall rigidity of the emulsion gel. These results help rationalize some of the diverse structures that are observed upon flocculation of the more usually studied polydisperse emulsions.

Materials and Methods

The crude emulsion is formed from silicon oil droplets in water, stabilized by sodium dodecyl sulfate (SDS). Highly monodisperse emulsion droplets, about 0.6 pm in diameter, are obtained by repeated crystallization fractionation. The surfactant concentration in the continuous phase is adjusted to about 0.01 M, slightly above the critical micelle concentration, ensuring the stability of the emulsion. To induce adhesion between the droplets, sodium chloride is added to the continuous phase.

Results

The strength of the adhesion is strongly temperature dependent: Above an onset temperature, T, the droplets remain repulsive, whereas adhesion is induced between the droplets when the temperature is reduced below T, and the strength of the adhesion increases as the temperature is further decreased. We believe that this may be associated with the formation of a nonzero contact angle, phi, between droplets, as has been observed for larger, polydisperse emulsion droplets. The onset temperature depends sensitively on the concentration of sodium chloride, increasing with increasing NaCl concentration. All experiments are carried out by rapidly lowering the temperature several degrees below T, to induce the attraction, ensuring strong adhesion between the droplets. The structure of the flocculating emulsions are directly observed using a phase contrast microscope with a variable temperature stage.

While the temperature of the emulsion is maintained above T,, the droplets undergo random Brownian motion, and there is a sufficiently large, short-range repulsive barrier between them to ensure stability against flocculation. When the temperature is lowered several degrees below T, a strong adhesion develops between the droplets. However, while the droplets adhere to one another, they retain their integrity and do not coalesce.


The gel also depends on the contact angle phi. At phi = 0.005, the structure of the gel at short length scales consists of a highly random, tenuous network of droplets. The distinct branches of the network are roughly a single droplet in thickness. As phi is increased to 0.01 and then to 0.05, the average thickness of the branches increases slightly, but the structure still exhibits a random, tenuous network at short length scales. As phi is increased still further, the average thickness of the branches increases substantially and the tenuous network is no longer observed at short length scales. Instead, the gel has a dramatically different appearance, with a random, weak and free-flowing network.

Discussion

A remarkable feature of these emulsion gels is the fact that they form a tenuous network, which must maintain some inherent rigidity, despite the fact that it is composed entirely of liquid droplets. This rigidity has a profound implication about the nature of the adhesion: It suggests that there must be no slip between two adjacent droplets. If slip could occur, the droplets would be able to rotate about their bonds, and the resultant rearrangement would lead to a significantly less tenuous structure. This suggests that a rigid film is formed by the adhesion between neighboring droplets. One possibility is that the surfactant forms a solid layer at the interface, which prevents re-arrangement of the atoms on the surface.

Soft Matter Applications

Under favorable conditions emulsification offers the opportunity to incorporate almost any ratio of one fluid in another. This leads to a wide range of important technological applications, ranging from cosmetics to coatings and from foods to medicines. Emulsions are also fascinating dispersions of deformable spheres, which exhibit novel colloidal properties. For example, the interaction between droplets can be made strongly adhesive, ultimately leading to a nonzero contact angle between droplets. If the droplets retain their integrity, this adhesion causes flocculation of the emulsion droplets. Emulsion flocculation, or creaming, is a commonly encountered phenomenon. If the phenomenon is understood, it could be used in many different colloidal applications.