Themed issue: modelling of soft matter

“The use of computation is the third major scientific theme that will push chemistry in the future. Computational power is one of the few items in chemistry that is rapidly becoming more available. Large-scale computation has traditionally been the province of the “quantum mechanician”. Computation (especially in semiempirical forms such as molecular mechanics, molecular dynamics, and large-scale simulation) is now rapidly becoming an indispensable component of experimental programs as well, particularly those concerned with complex systems.” So wrote George Whitesides almost twenty years ago.¹ Since that time the explosion of cheaply available computer processing power, the improvement of existing simulation methodologies, new techniques for mesoscale simulation, and the development of effective parallelisation for computer codes, have led to computer modelling becoming a key tool for the study of many soft matter systems. Today, computational chemists and physicists are turning to complex phenomena that emerge only on mesoscopic or even microscopic scales.

Soft matter modelling crosses many traditional boundaries. For example, the design of new nanostructured soft materials borrows heavily from biology, from liquid crystals and from polymers. Chemists and physicists are using molecular interactions to engineer microphase (or nanophase) separation and form well-ordered nano-domains for new applications.² In recent times modelling has shed light on such diverse properties as self-assembly processes in micelles and vesicles,³ mesophase formation in polymers and amphiphilic systems,⁴ the formation of different liquid crystal phases,⁵ supramolecular assembly, and dynamical processes in colloidal dispersions.⁶

An interesting common feature of all these soft matter systems is that insight is often provided by viewing systems at different length scales. Today studies are carried out over scales ranging from atomistic, to coarse-grained models (where some of the chemical details can be dispensed to provide a simpler off-lattice or on-lattice description); to lattice Boltzmann methods; and models which provide a continuum description.

It was the explosion of interest across the range of soft matter modelling that led this year to a Faraday Discussion on the topic of multiscale modelling of soft matter. Faraday Discussion 144 in Groningen brought together researchers from the different (but related) fields of liquid crystals, polymers, colloids, membranes and self-assembled systems, alongside colleagues who were working on methods designed to better cross the diverse time and length scales associated with soft matter systems.

This themed issue of Soft Matter has been published to coincide with the published volume of Faraday Discussion 144. It brings together a series of outstanding papers, which cover the diverse range of topics that comprise soft matter modelling in 2009.

The key issue of crossing scales is central to a number of papers. The review by Peter and Kremer [DOI: 10.1039/b912027k] addresses exactly this problem, looking at the way in which coarse-grained (CG) simulation models can be linked to a higher resolution atomistic description. The review looks at the possibility of going from the atomistic to the CG descriptions (and back again) to provide long time- and length-scale simulation data for comparison with experiment. Addressed also are the future challenges towards true multi-scale modelling provided by adaptive resolution models; which (in principle at least) may in future allow simulators to work seamlessly between the atomistic, coarse-grained and (even) continuum regimes. Classic examples of crossing between atomistic and coarse-grained descriptions are provided by Fritz et al. [DOI: 10.1039/b911713j] who consider permeation of molecules in polymer melts, and by the paper of Padding et al. [DOI: 10.1039/b911329k] who review their recent work on the development of a multi-scale simulation methodology to calculate the rheology and flow of wormlike micelles.

Considerable insight into the behaviour of soft matter systems under flow can be provided by a number of different simulation methodologies. This themed issue provides some intriguing insights for different length scales from a series of reviews and papers. Van der Sman [DOI: 10.1039/b915749m] reviews the case of suspension flows at multiple length scales, McLeish et al. [DOI: 10.1039/b916288g] review the progress made in the multiscale models designed to understand molecular rheology in polymers; while the Briels review [DOI: 10.1039/b911310j] looks at the treatment of transient forces that arise in coarse-graining soft matter systems under flow. Cubic phases of amphiphiles under shear flow are considered by Saksena and Coveney [DOI: 10.1039/b911884e] by means of a kinetic lattice Boltzmann model; mesoscale simulations are used to look at the velocity flux needed to push a polymer into a narrow channel by Yeomans et al. [DOI: 10.1039/b909208k]; and Hanna et al. [DOI:10.1039/b916087f] consider the relaxation of a tethered polymer under shear flow.

One of the outstanding developments in recent years is that soft matter simulation is starting to provide a lead to experimentalists in terms of the prediction of new structures and materials, which have never been seen before. We see this in the paper from the Earl group in Pittsburgh [DOI: 10.1039/b911359b], where new chiral superstructures and phases are formed from the interaction of achiral particles. Beautiful experimental evidence of structures formed by chiral segregation in mesophases has recently been provided by back-to-back experimental papers by Hough et al. in the journal Science^7,8 and an intriguing paper on achiral bent core mesogens by Görtz et al. in Soft Matter.⁹ Novel self-assembled structures are shown in the coarse-grained modelling approaches from the Zannoni [DOI: 10.1039/b911336c], and Glotzer groups [DOI: 10.1039/b909669h]. These papers are written from liquid crystal and block copolymer perspectives but both are able to achieve long-range ordering of tethered nanospheres. A slightly different approach to self-assembly is provided by Daoulas et al. [DOI: 10.1039/b911364a], where a novel mesoscopic density functional theory-based Monte Carlo approach is used to study self-assembly in multi-component systems of conventional macromolecules and supramolecular entities.

A fast lattice Monte Carlo (FLMC) method is introduced in the paper by Wang [DOI: 10.1039/b909078a]. This paper provides an efficient way of simulating soft matter systems, which provides an efficient alternative to conventional lattice Monte Carlo or fast off-lattice Monte Carlo work.

As ever, modelling studies can throw up surprising and intriguing discoveries. This is seen in the paper by van Teeffelen et al. [DOI: 10.1039/b911365g] who show that a self-propelled rod (a real-life example could be a bacterium), driven by a constant internal force and torque, which initially performs circular clockwise motion will undergo anti-clockwise motion when confined to a ring. Intriguing results are also seen in the work of Ravnik and Žumer [DOI: 10.1039/b913065a], who consider the assembly of colloidal particles in nematics.

In the last few years, modelling has made a big impact on how scientists view membranes. This issue includes four papers on lipids, with Xing and Faller looking at the differences between supported and unsupported monolayers [DOI: 10.1039/b912719d]. Risselada and Marrink consider the liquid crystal–gel transition in lipid vesicles [DOI: 10.1039/b913210d], and Olmsted et al. look at the water permeability through bilayers [DOI: 10.1039/b911257j]. A review by Vattulainen et al. looks at the interaction of carbon nanoparticles with lipid membranes [DOI: 10.1039/b912310e]

This themed issue would not be complete without a series of four papers, which attempt to use modelling to provide fundamental insights into some very difficult problems! Papadopoulos et al. [DOI: 10.1039/b911159j] attempt to understand the interconnection of the nanocrystal and amorphous phases of spider dragline silk, Lenz et al. [DOI: 10.1039/b911357f] look at the adsorption of dendrimers onto colloidal particles; Vink [DOI: 10.1039/b912135h] reviews critical behaviour in bulk and random porous media; and Cerdà et al. [DOI: 10.1039/b912800j] critically review the attempts made to model the structure and dynamics of polyelectrolyte multilayers.

It is tempting to speculate where soft matter modelling will take us over the next 20 years. Continued improvement in GUI design and more user-friendly programs will dramatically increase the usability of modelling software. So we will certainly see some of the cutting edge developments of today becoming accessible to chemists and physicists at the desktop, much in the way that single-molecule molecular mechanics and quantum mechanics methods have already done so. It is also likely that available computer power will continue to grow; not necessarily through dramatically faster chips, but certainly through multi-core technology and other forms of parallel processing. This improvement in computational power will make a range of new problems accessible. Coarse-grained models will continue to evolve and over the next few years we should expect them to shed more light on phenomena occurring on long time- and length-scales. Probably the most exciting developments will be in the methodologies designed to cross the time- and length-scales. We are still learning how to do this well but, as some of the papers in this themed issue point out, the future looks promising.

My thanks go to all the authors who have taken the time and effort to assemble this themed issue. My thanks also to the RSC staff for their guidance, and professionalism. Together with the published volume of Faraday Discussion 144, this issue provides a cutting edge picture of the state of play in this exciting field of research. Here's to the next 20 years!

Mark Wilson, Durham, UK

References

1. G. Whitesides, What Will Chemistry Do in the Next Twenty Years?, Angew. Chem., Int. Ed. Engl., 1990, 29, 1209–1218.
2. F. A. Detcheverry, D. Q. Pike, P. F. Nealey, M. Müller and J. J. de Pablo, Simulations of theoretically informed coarse grain models of polymeric systems, Faraday Discuss., 2010 10.1039/b902283j.
3. S. J. Marrink, D. P. Tieleman and A. E. Mark, Molecular dynamics simulation of the kinetics of spontaneous micelle formation, J. Phys. Chem. B, 2000, 104, 12165–12173.
4. F. J. Martínez-Veracoechea and F. A. Escobedo, Monte Carlo study of the stabilization of complex bicontinuous phases in diblock copolymer systems, Macromolecules, 2007, 40, 7354–7365.
5. M. R. Wilson, Molecular simulation of liquid crystals: progress towards a better understanding of bulk structure and the prediction of material properties, Chem. Soc. Rev., 2007, 36, 1881.
6. C. N. Likos, Soft matter with soft particles, Soft Matter, 2006, 2, 478–498.
7. L. E. Hough, M. Spannuth, M. Nakata, D. A. Coleman, C. D. Jones, G. Dantlgraber, C. Tschierske, J. Watanabe, E. Körblova, D. M. Walba, J. E. Maclennan, M. A. Glaser and N. A. Clark, Chiral Isotropic Liquids from Achiral Molecules, Science, 2009, 325, 452–456.
8. L. E. Hough, H. T. Jung, D. Krüerke, M. S. Heberling, M. Nakata, C. D. Jones, D. Chen, D. R. Link, J. Zasadzinski, G. Heppke, J. P. Rabe, W. Stocker, E. Körblova, D. M. Walba, M. A. Glaser and N. A. Clark, Helical Nanofilament Phases, Science, 2009, 325, 456–460.
9. V. Görtz, C. Southern, N. W. Roberts, H. F. Gleeson and J. W. Goodby, Unusual properties of a bent-core liquid-crystalline fluid, Soft Matter, 2009, 5, 463–471.

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