Soft Matter Blog

Example of nematic texture and defect structures observed in liquid crystals confined in micro-channels.

Can the flow of active and passive liquid crystals be accurately described using the same theoretical model? This was the question posed by Miha Ravnik, from the University of Oxford in a recent seminar. The motivation for this work is to improve the understanding of liquid crystal (LC) flow in micro-channels. The coupling of material flow with orientation in LC, via internal material stress, is very interesting both technologically, as a driving mechanism for controlling material flow, and for understanding the behaviour of artificial and biological swimmers. Flow is also important for understanding topological defects in LC (Soft Matter doi: 10.1039/B810933H).

The model of Ravnik is based on the phenomenological Beris-Edwards model, solved using a hybrid lattice-Boltzmann method. The equations couple the orientation, described by an order parameter, with the flow velocity field, which is modelled using a generalised Navier-Stokes description. The orientation describes the LC alignment in the flow, the molecular field and the internal motility of the LC (this is zero for passive LC). The results show that for both passive and active LC the behaviour is dependent on the magnitude of the driving flow and the dimensions of the channel. This includes the LC orientation and flow profiles and the position of defect lines.

Similar behaviour was recently observed experimentally by Sengupta et al. (Soft Matter doi: 10.1039/C1SM05052D), who studied the flow of nematic LC through micro-channels. Different textures and defect structures were observed to develop depending on the channel dimensions and the flow rate applied. Some very nice movies, showing the formation of these structures, can be found as supplementary information with the article.

Miha Ravnik is also interested in understanding the behaviour of topological defect loops seen when colloidal particles are added to liquid crystals (Soft Matter doi: 10.1039/B913065A). The controlled manipulation of these defect loops was discussed in a previous post.

A talk given by Ravnik on liquid crystal colloids can be found here. The talk presents routes for the functionalisation of colloidal particles and continuum liquid crystals.

Lipid bilayers, vesicles and lipid-polyelectrolyte phases forming on polyelectrolyte multilayers.

Sergio Moya is a soft matter scientist at the CIC biomaGUNE in San Sebastian, Spain. His research focuses on the nano-fabrication of soft matter for various applications including medical applications, to form membranes for water filtration and to study nano-toxicity. Moya is particularly interested in utilising polyelectrolytes as they are “easy to assemble, pattern and synthesise”.

Moya studies polyelectrolyte growth and behaviour using, amongst other things, a quartz crystal micro-balance in tandem with ellipsometry and atomic force microscopy. This allows the growth and assembly of polyelectrolyte multilayers to be monitored along with their mechanical properties and water content (doi: 10.1021/ma1015984).

Polyelectrolyte multilayers have a number of different uses including the non-covalent functionalisation of particles (doi: 10.1021/la803360n). Once coated with polyelectrolytes Moya has shown that the core particles can subsequently be removed without damaging the multilayer coating.  The multilayer can also be selectively removed or attached using the appropriate surfactants (doi:10.1021/jp908608u). Polyelectrolyte multilayers have also been used to support lipid bilayer membranes and study their formation (Soft Matter, doi: 10.1039/b805754k). Bilayers, absorbed vesicles and 3D lipid-polyelectrolyte phases have all been seen to form (see figure).

Finally, in a recent Soft Matter paper, not discussed in his talk, Moya has shown that the toxicity of carbon nanotubes can be reduced when coated with polyelectrolyte-lipid layers (Soft Matter, doi: 10.1039/C0SM01511C).

Sergio Moya recently gave a seminar at the Max-Planck Institute for Dynamics and Self-Organization as part of the Dynamics of Complex Fluids seminar series.

Six colloid particles entangled by two unlinked defect loops. Image courtesy of U. Tkalec.

“For the first time, knot formation has been fully controlled and rewired inside liquid crystals.” – Uroš Tkalec

From tying shoes laces, to knitting a jumper to securing a boat, knots are ubiquitous and important in many aspects of everyday life. Knots are also of interest scientifically. Knots have been engineered to inhibit enzymes crucial in infectious diseases (doi:10.1039/B801667D). Semi-flexible polymer chains can be made to form a figure of eight (doi:10.1039/C0SM00290A), while pseudo knots in helical chains can result in stable entanglements that can be built and destroyed (doi:10.1039/B719234G). Even chocolate can be formed in such a way that it is flexible enough to be tied in knots or coiled into a spring (doi:10.1039/B518021j).

Uroš Tkalec from the Jožef Stefan Institute in Slovenia and coworkers have taken the study of knots one step further. In their paper, recently published in Science, the group used laser tweezers to manipulate liquid crystal-colloid mixtures forming knots and links.

When added to a liquid crystal, colloid particles disrupt the crystal ordering creating microscopic topological defect loops. Tkalec manipulated these defects loops using laser tweezers to create loops and knots of arbitrary complexity. Knots demonstrated in the paper include the trefoil, pentafoil and the granny knot.

“The knots and links created here are a rare, potential implementation of mathematical knot theory”- says Tkalec. These knots have potential applications in soft photonic materials, for the control of light in optical liquid crystal microcircuits. Tkalec suggests that their results may also be of relevance in understanding non-trivial topological entities in a number of soft matter systems such as polymers, DNA and proteins.

In the last two blogs, I reviewed the top ten most cited papers in Soft Matter. Now I want to move on to the most cited articles in Soft Matter of all time (well, since 2005 when the journal was founded).

The top ten articles (as determined by ISI Web of Knowledge) are listed below. Their subject matter is diverse and ranges from why materials wrinkle and buckle, to the self-assembly of Janus particles, to a new experimental technique to measure the yield stress of fluids. However, despite this diversity, one topic clearly stands out in the top ten: superhydrophobicity. The top two spots are occupied by reviews on water repellency and superhydrophobic surfaces. Progress on superhydrophobic development by Roach, Shirtcliffe and Newton featured in an earlier blog and was also one of the most read articles in Soft Matter during 2010. A discussion on the measurement of contact angles also features in the top ten.

At number 1… On water repellency by Mathilde Callies and David Quéré is a review article looking at the physical mechanisms responsible for water repellency. It includes a discussion of switchable wettability and the dynamic properties of droplets on superhydrophobic surfaces. However, it is the questions that the authors pose, which are perhaps most interesting.

The authors start by stating that measurement of a single contact angle is not sufficient to characterise the wettability of a surface. A single contact angle does not give any information about how that droplet sits on the surface; is it the Cassie-Baxter or the Wenzel state? It also doesn’t provide any information on the ‘stickiness’ of the surface. To fully characterise a material contact angle hysteresis measurements must be carried out.

Quéré and Callies suggest that in addition to contact angle hysteresis, three complementary measurements should be made to answer the following questions: (1) What is the maximum radius a drop can have before it will roll off a surface inclined at a given angle? (2) What is the critical pressure required to change a droplet from being in the Cassie-Baxter state to being in the Wenzel state? (3) What is the threshold velocity, below which an impacting droplet will stick to the surface rather than bouncing off? The authors believe that this data would allow different surfaces to be more reliably compared.

The paper concludes with the statement that many questions remain unanswered regarding water repellent surfaces, in particular with respect to optimisation of the surfaces. The following are a selection of the questions that the authors pose: How does superhydrophobicity vary as a function of surface texture? How can we optimise a given material or design? Can special designs be used to get special properties? What is the maximum texture size/density required to promote water repellency? How can we make self-cleaning water repellent materials more robust?

On water repellency was published in the first issue of Soft Matter back in 2005. The citations show a huge development in the understanding of superhydrophobic surfaces over the last six years. There has been an explosion in the number of different water repellent surfaces and structures that can be fabricated. These include triangular polyimide pillars, hierarchical bio-fibres, chemically roughened aluminium, copper and zinc, structured teflon and silicone nanofilaments to name but a few. Surfaces have been designed allowing for tuneable adhesion of water (see for example Lai et al. and Di Mundo et al.). Yeh, Chen and Chang have studied how pillar size and spacing changes the wetting properties of the surface. They show that surface coverage and surface roughness strongly influence the hysteresis behaviour. Robustness of the surfaces has also been improved through material choice and pattern design.

With 220 citations to On water repellency I could keep going and going on the developments in the field. This selection shows that while a number of points raised by Quéré and Callies have been addressed (at least partially) over the last six years, some remain unanswered.

Top ten of all time:

• On water repellency. Callies M, Quéré D (2005). Citations: 220*.
  
• Progress in Superhydrophobic surface development. Roach P, Shirtcliffe NJ, Newton MI (2008). Citations: 211.
• Soft matter with hard skin. Genzer J, Groenewold J (2005). Citations: 134.
• Janus particles. Walther A, Muller AHE (2008). Citations: 130.
• Peptide based stimuli-responsive biomaterials. Mart RJ et al. Citations: 128.
• Surface-initiated polymerisations from silica nanoparticles. Radhakrishnan B, Ranjan R, Brittain WJ (2006). Citations: 94.
  
• Soft contact: measurement and interpretation of contact angles. Marmur A (2006). Citations: 94.
• Synthesis and applications of emulsion-templated porous materials. Zhang HF, Cooper AI (2005). Citations: 93.
• Bioinspired functional block copolymers. Borner HG, Schlaad H (2007). Citations: 92.
• Yield stress and thixotrophy: on the difficulty of measuring yield stress in practice. Moller PCF, Mewis J, Bonn D (2006). Citations: 87.

*Citation numbers taken on the 15th June 2011.

This is the second in a series of blogs looking at the most cited articles in Soft Matter. The first part can be found here. The countdown continues with the top five in the top ten 2010.

At number 5… In fifth place, with 14 citations, is ‘Block copolymer multiple patterning integrated with conventional ArF lithography‘. As technology continues to miniaturise, patterning on the nanometre scale (sub 30 nm) is no longer just desirable, but a requirement for semiconductor devices and chips. In this article, Park et al. combine photolithography with the self-assembly of block copolymers to create patterns down to 20 nm in size.

Initially, the photoresist was patterned using the industry standard 193 nm ArF lithography to create an asymmetric pattern. An underlying polymer brush layer was then oxidised and the photoresist washed away. The result was a chemical asymmetric pattern of alternating neutral and polar stripes. The block copolymer was spun onto this and self-assembled to create arrays of standing cylinders aligned with the underlying pattern. Examples can be seen in the image above. These block copolymers were then used as templates to create arrays of nanodots and nanopillars. This method provides a simple way to pattern on the sub 20 nm scale and avoids multiple photolithography steps or switching to shorter wave-length light sources.

The citing articles all look at patterning on the nanoscale. One article worth highlighting is that of Lee et al. published in Adv. Funct. Mater. This article reviews tailored and self-assembly of carbon nanotubes and graphene to form 3D patterns. Park’s block copolymer templates offer a way to direct the growth of nanotubes into well-ordered arrays.  Graphene has also been used to chemically modify surface energy. Deposition of block copolymers onto these graphene films results in highly aligned perpendicular lamellae, which can then be used for patterning.

Difference is number and morphology of cells for soft and hard surfaces.

At number 4… Next up, with 15 citations, is ‘Spatially controlled hydrogel mechanics to modulate stem cell interactions‘. This is the third (but not final) appearance of hydrogels in the top ten 2010. The focus of the paper is on how hydrogel properties affect the spreading and proliferation of human mesenchymal stem cells. Understanding this behaviour has importance and relevance in tissue engineering and cellular behaviour in disease states.

The morphology and proliferation of the stem cells were seen to be highly dependent on the underlying hydrogel. Increased spreading and growth was seen for stiffer hydrogels. Wang et al. have gone on to show that stiffer surfaces also result in more organised cytoskeletons, more stable focal adhesion and faster migration of the stem cells. In addition, gene expression was seen to differ for cells cultured on soft and hard hydrogels. On the softer hydrogels, neuron specific proteins were expressed by the cells, while on stiffer hydrogels there were not. Instead the cells expressed myogenic proteins. Very recently, Huang et al. proposed a mechno chemical coupling model to explain the dependence of spreading and adhesion on substrate stiffness.

At number 3… In 3rd place is ‘Nematic phases of bent core mesogens‘ with 16 citations. This paper by Keith et al., looks at the chain-length and temperature dependence of the phase behaviour of bent core mesogens. These mesogens, derived from 4-cyanoresorcinol with terminal alkyl chins, display broad metastable nematic phase ranges at ambient temperatures and do not have smectic low temperature phases. The authors suggest that mixtures of these compounds may potentially give rise to bent-core materials with stable nematic phases at room temperature.

At number 2… In second place is ‘In pursuit of propulsion at the nanoscale‘ by Stephen Ebbens and Jonathon Howse with 20 citations. Both authors are both currently at the University of Sheffield, UK. This review article looks at the developments in self-propelling nano and micro-scale swimming devices, with emphasis on swimming transporters. Ideally these swimmers should operate independently, without an external stimulus. Ebbens and Howse focus on bi-metallic nano-rod swimmers, which have been demonstrated to transport cargo. This was also discussed in a mini-review by Pumera.

Tierno et al. have shown that paramagnetic catalytic microellipsoids can be guided using external magnetic fields. Their particles align along the field and are seen to travel perpendicularly to the field in an almost straight trajectory. For spherical particles this is not the case.  Sanchez et al. have developed autonomous hybrid biocatalytic microengines. These engines use enzymes as their catalysts and are able to achieve speeds of 10 body lengths per second. In a second paper, Sanchez et al. demonstrated the transportation of cells using microbots. However, while the microbots were self propelled, loading transporting and delivery of the cells was externally controlled. I should mention that the two papers by Sanchez discuss very different swimmers. It is clear that this is currently a very hot area or research, due in part to their wide ranging potential medical applications.

Number 1… And finally, at the top of the top ten 2010 is ‘Responsive reversible hydrogels from associative “smart” molecules‘, published online in February 2010. The review article by Constantinos Tsitsilianis looks at the reversible hydrogels through self-assembly and association mechanisms.

Inverse-trapezoidal superhydrophobic structure.

Since its birth in 2005, Soft Matter has quickly grown to be one of top (if not the top) journals in its field. The current impact factor of the journal is 4.87. In this and the next few blogs, I thought it would be interesting to explore the themes and articles being published in Soft Matter and their impact on the soft matter field. Obviously I can’t, nor would you want me to, discuss every article published in Soft Matter, so I will limit the discussion to the most cited articles (as determined by ISI Web of Knowledge). What are the hot topics? Who is writing these highly cited articles and who is citing them? In this first blog of the series, I will start with the top ten of 2010 and later the top ten of all time (well, since 2005).

Top ten 2010 (numbers 10-6)

At number 10… In tenth place, with 10 citations since its online publication in Feb ’10, is ‘A robust superhydrophobic and superoleophobic surface with inverse trapezoidal microstructures on a large transparent flexible substrate‘. In the article the authors Maesoon Im et al., from KAIST South Korea, demonstrate a large size flexible transparent PDMS microstructured surface, which is both oil and water repellent. Im et al. structured the surface of the PDMS to create an array of inverse-trapezoidal microstructures (see image).

Of the articles citing this work, three build on it directly. The authors look at the use of these structures for combined electrical insulation and water repellence (doi:10.1021/la101339t) and also their self-cleaning properties for potential solar cell applications (doi:10.1039/c0jm02463e). One important criterion for useable superhydrophobic surfaces is their durability. This was not discussed by Im et al. However, Su et al. have shown that similar superhydrophobic polyurethane elastomeric surfaces have high abrasion resistance, making them technologically very interesting.

This isn’t the first blog that I have written looking at superhydrophobic surfaces and it is likely not the last. Anything with the potential to make our lives easier (via self-cleaning), reduce energy consumption (via friction reduction) and enhance energy capture (by remaining dust free) is always going to be in the spotlight.

At number 9… In 9th place, also with 10 citations, is ‘Insights into the cybotactic nematic phase of bent core molecules‘. The article, written by Francescangeli and Samulski, was published online in April 2010. For those not in the liquid crystal field the term cybotactic here describes an assembly of molecules in a nematic mesophase, which are arranged in a short-range smectic-like array.

At number 8… With 11 citations, ‘Why are double network hydrogels so tough?’ makes it to number 8 in the top ten 2010. This review article by Jian Ping Gong, Hokkaido University Japan, provides an interesting overview of double network hydrogel research with particular emphasis on understanding their mechanical properties.

Hydrogels appear four times in the top ten 2010. One of the reasons for this is perhaps their suitability as artificial soft tissues, due to their high water content and high mechanical strength and toughness. From the citations to this article it can clearly be seen that there is a strong focus on enhancing the strength, durability and bonding of hyrodgels in aqueous environments. Also important for these biological applications, is that the hydrogel is biocompatible. This was highlighted in one of Gong’s most recent citations, where Bai et al. fabricated high strength fatigue resistant hydrogels with low cytotoxicity and antifouling properties. This hydrogel is not itself biodegradable.  Bai et al. claim however, that their method could be easily adapted to form a biodegradable, biocompatible high-strength hydrogel with applications for tissue engineering scaffolds.

At number 7…Since its publication in March 2010, ‘Relationship between the molecular structure, gelation behaviour and gel properties of Fmoc-dipeptides‘ has received 12 citations, putting it at number 7. The second appearance for hydrogels, the focus of this paper is on how the chemical structure of the gelator (in this cast Fmoc-dipeptides) relates to the properties of the gel formed. The number of the amino acids in the dipeptide was varied allowing the hydrophobicity of the molecule to be tuned. Depending on the hydrophobicity different types of gel were seen to form.

Of the 12 citations for this article, all looked at various aspects of dipeptide gelation. In contrast to the double network hydrogels where strength and durability were key, for this class gel formation of the gel structure is reversible. The structure in the gel arises from the entanglement of the fibres via hydrogen bonding  or π-π stacking, rather than cross-linking in the conventional sense (covalent or ionic bonding). This makes them attractive for drug release and wound treatment.

At number 6… And finally (for this blog), in 6th place is ‘Dendrons/dendrimers: quantized nano-element like building blocks for soft-soft and soft-hard nano-compound synthesis’. This tutorial review has received 13 citations since its online publication in December 2009 (not strictly 2010, but since it appears in the 2010 print edition it counts).

Tomalia defines dendrimers as “well defined collections of atoms that can be viewed as core-shell type atom mimics”. The focus of the review is to show that dendrons/dendrimers are emerging as a platform for synthetic nano-chemistry. This is reflected in the citations, which include papers on the use of dendrons/dendrimers for gene delivery, as biocompatible drug delivery vehicles, in blue LEDs and as self-assembling building blocks.