Difference between revisions of "Phase Diagram and Effective Shape of Semiflexible Colloidal Rods and Biopolymers"

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'''Journal:''' PRL 106, 208302 (2011)
 
'''Journal:''' PRL 106, 208302 (2011)
  
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'''Keywords:''' [[self-assembly]], [[liquid-crystal]], [[virus]], [[colloid]]
  
 
== Summary ==
 
== Summary ==
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The authors then cite a theoretical model by Turner and Cates which suggests that not only is high shear necessary for micelle formation, but that highly-aligned flow is also necessary. Thus, the authors employed a microfluidic setup that contained glass bead, forming a porous network. Therefore, flow through the beads is highly ordered but also is strong enough to induce micelle formation. Figure 2 shows different stages of the gel/micelle formation and its interaction with the packed beads. Long, stable gels can be seen. It should be noted that the authors were able to store such gels for months without seeing degradation. This is a marked improvement over previous methods by which the gels immediately dissociated.  
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Figure 2 shows the effective shape for a mixture of thick-thin fd virus particles throughout the phase diagram. It is immediately apparent that throughout the phase diagram, while the rods always behave as shorter,thicker rods, the effective shape varies considerably. Interestingly, the thick rods stretch out more than the thin rods, with the rods now effectively differing in both diameter and length. The authors conclude that  that a fixed effective shape does not capture the essential physics of these suspensions; the state-point dependent stretching of the flexible rods is a key feature.
  
 
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Finally, the authors characterized these nanogels to ensure that they properties are suitable with the new manufacturing method. CryoTEM was used to observe the nanostructure of the material, which is given in the figure below. It can be seen that the average micelle width was 28nm and that the average inter-micelle spacing was 41nm.
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Finally, the authors studied worm-like micelles (Figure 3) in the I-N phase transition for different micelle models. In all, the incorporation of flexibility into the model improved the correlation of experimental results to the model as opposed to using rigid rods. Essentially, the effective shape of the particles is allowed to vary across the phase diagram.  
  
 
[[image:Darnell6_3.jpg|thumb|center|350px]]
 
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== Connection to Soft Matter ==
 
== Connection to Soft Matter ==
  
Gels are a very common focus of research in soft matter physics due to their applications in biomedicine, photonics, and sensing. The properties of these gels, however, are often dificult to tailor and require caustic chemicals that prevent their use in biological systems. Many of the current biomaterials used in biomedicine are micro or even macroporous, and the space of nanoporous gels has not been deeply explored. This paper's use of purely physical means to create robust nanoporous gels suggests the potential viability of using nanogels in biological applications. From a manufacturing perspective, this paper highlights one of the more successful broad applications of microfluidics, that being the ability to capitalize on low Reynolds-Number flows to manipulate structures on scales that otherwise were only tractable via chemical means.
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Phase diagrams represent one of the most important tools in soft matter physics, since understanding of a phase diagram allows for the effective processing of different mixtures and materials. The system of interest, f n virus particles, is an increasingly interesting system as the field becomes more bio-oriented. In addition, this paper raises the question of applying similar ideas of introducing flexibility into models into inhomogeneous systems. The prospect of assessing the phase behavior of inhomogeneous systems, especially those which are biologically relevant, is extremely interesting since the mechanical interactions of biological systems are being increasingly appreciated.

Latest revision as of 15:10, 30 November 2011

Entry by Max Darnell, AP 225, Fall 2011

Reference:

Title: Phase Diagram and Effective Shape of Semiflexible Colloidal Rods and Biopolymers

Authors: M. Dennison, M. Dijkstra, and R. van Roij

Journal: PRL 106, 208302 (2011)

Keywords: self-assembly, liquid-crystal, virus, colloid

Summary

Rodlike particles show great promise in bioengineering due to their ability to self-assemble into a wide variety of liquid-crystal phases. One of the most important systems is f n viruses, which can be engineered into needles by incorporating a polyethylene-glycol (PEG) coating. Unfortunately, however, these phase diagrams of such systems, and the mechanisms behind each phase, have been poorly understood. This paper identifies the key parameter in the state of the system as flexibility of the needles. Also, the stretching of micelles formed from these needles is examined.

Methods/Results

The authors followed a segmented-chain model first introduced by Wessels and Mulder in which flexibility is incorporated into the model by introducing a bending potential parameter between segments in the chain. Also, the excluded volume is taken into account. The model is then generalized to two-component systems and the full phase behavior is mapped out. Also, the effective particle shape is calculated, which turns out to be strongly state-dependent and thus crucial in building phase diagrams. The authors calculated the phase diagrams for mixtures of bare fd particles (species 1, thin) and PEG-coated ones(species 2, thick), with equal contour and persistence lengths.

Darnell6 1.jpg

Figure 2 shows the effective shape for a mixture of thick-thin fd virus particles throughout the phase diagram. It is immediately apparent that throughout the phase diagram, while the rods always behave as shorter,thicker rods, the effective shape varies considerably. Interestingly, the thick rods stretch out more than the thin rods, with the rods now effectively differing in both diameter and length. The authors conclude that that a fixed effective shape does not capture the essential physics of these suspensions; the state-point dependent stretching of the flexible rods is a key feature.

Darnell6 2.jpg

Finally, the authors studied worm-like micelles (Figure 3) in the I-N phase transition for different micelle models. In all, the incorporation of flexibility into the model improved the correlation of experimental results to the model as opposed to using rigid rods. Essentially, the effective shape of the particles is allowed to vary across the phase diagram.

Darnell6 3.jpg

Connection to Soft Matter

Phase diagrams represent one of the most important tools in soft matter physics, since understanding of a phase diagram allows for the effective processing of different mixtures and materials. The system of interest, f n virus particles, is an increasingly interesting system as the field becomes more bio-oriented. In addition, this paper raises the question of applying similar ideas of introducing flexibility into models into inhomogeneous systems. The prospect of assessing the phase behavior of inhomogeneous systems, especially those which are biologically relevant, is extremely interesting since the mechanical interactions of biological systems are being increasingly appreciated.