Top ten 2010 continued…

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.

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Hot articles: Gel swelling, viscous drag friction and microdomain orientation…

Read these Hot Articles for free until June 24:

viscous drag frictionFernandez-Nieves and co-workers describe the use of hydrostatic pressure to change the size and structure of poly-(N-isopropylacrylamide) based microgel particles. These changes rely on changes in the miscibility of the polymer in the solvent. Pressure was found to play an analogous role to temperature in swelling of the gels. Juan José Liétor-Santos, Benjamín Sierra-Martín, Urs Gasser and Alberto Fernández-Nieves, Soft Matter, 2011, DOI: 10.1039/C1SM05328K

viscous drag frictionThe viscous drag friction acting on fluid drops in confined geometries is a fundamental issue when dealing with small amounts of liquid. Researchers have established unexpectedly simple scaling laws for viscous drag friction in different regimes, replacing the well-known Stokes’ friction law. Ayako Eri and Ko Okumura, Soft Matter, 2011, DOI: 10.1039/C0SM01535K

 

block copolymer thin filmSupramolecular assembly is demonstrated to efficiently control the microdomain orientation in block copolymer thin films. The effect of low-molecular-weight additives on the microdomain orientation was investigated. At low grafting density the microdomains are parallel to the surface, while above a critical grafting density they switch to perpendicular.  Chih-Hung Lee and Shih-Huang Tung, Soft Matter, 2011, DOI: 10.1039/C1SM05334E

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Hot Articles: magnetic flux lines, crystal nucleation and interfacial investigations….

Read these Hot Articles for free until June 22:

Magnetic flux lines

Magnetic flux lines were accurately modelled and mapped out in a new study on the effect of applied external fields to magnetic nanoparticle assembly. Ferrimagnetic iron oxide particles were used in the fossilized liquid assembly method, and agglomeration and sedimentation of the magnetic nanoparticles were also investigated . Ryan Schmidt, Jason Benkoski, Kevin Cavicchi and Alamgir Karim, Soft Matter, 2011, DOI: 10.1039/C1SM05057E

NucleationQuestions regarding heterogeneous nucleation in colloidal model systems are answered in this recent study. Although classical theories exist for crystal growth, the actual experimental determination of this process is still a challenge. Heterogeneous and homogeneous nucleation rate densities were both determined and compared. Andreas Engelbrecht, Rafaella Meneses and Hans Joachim Schöpe, Soft Matter, 2011, Advance Article, DOI: 10.1039/C1SM05246B

surfactantNew investigations into interfacial properties of surfactant-decorated particles in interfacial layers have been performed. Nanometric dispersions of colloidal silica nanoparticles with a cationic surfactant, CTAB were studied, and surfactant rheology was studied on a broad frequency window for the first time. L. Liggieri, E. Santini, E. Guzmán, A. Maestro and F. Ravera, Soft Matter, 2011, Advance Article, DOI: 10.1039/C1SM05257H

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Soft Matter poster prize winners at the 7th Annual European Rheology Conference

Congratulations to Carolina Vannozzi at the University of California, Santa Barbara, USA, for winning the Soft Matter poster prize at 7th Annual European Rheology Conference in Suzdal, Russia. Carolina Vannozzi’s poster was on “An analytic expression for the disjoining pressure between particle-stabilized fluid-fluid interfaces and composite materials”.

The two Soft Matter runners up were Valery Karbushev at A. V. Topchiev Institute of Petrochemical Synthesis, RAS (TIPS RAS), Moscow, Russia, with his poster “Physical-mechanical and rheological properties of composites based on thermoplastic and nanodiamonds” and Sergey Litvinov at Technical University Munich, Germany with “Smoothed dissipative particle dynamics model for polymer solutions“.

 Soft Matter Poster Prize winners

From left to right are Sergey Litvinov, Valery Karbushev and Carolina Vannozzi. Congratulations to all three winners.

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Top ten 2010

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.

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Hot articles: Wetting, self-assembly and Amyloid fibrils.

tear filmRheology in eye wetting: Wetting phenomena in the tear film, the coating of the eye, was modelled with four insoluble surfactants, including meibum, a primary component of the lipid layer of the tear film, on a water droplet. The work suggests that surface rheology has a strong influence on the wetting phenomena. Danielle L. Leiske, Cécile Monteux, Michelle Senchyna, Howard A. Ketelson and Gerald G. Fuller, Soft Matter, 2011, Advance Article, DOI: 10.1039/C1SM05231D

Self-assembled nanoparticlesExternal control of self-assembled nanoparticle size: Cyclodextrin was used as an external trigger to direct the size of self-assembled nanoparticles. The host molecule was added to polyadimoamine dendrimer and diazo dye nanoparticles to cause stepwise size increase of the particles. This allowed dynamic size control without the need for synthesis of specific building blocks, or direct inclusion of the cyclodextrin. Immanuel Willerich, Torben Schindler, Helmut Ritter and Franziska Gröhn, Soft Matter, 2011, Advance Article, DOI: 10.1039/C1SM05405H

Amyloid fibrilAmyloid protein pitch: New experimental and theoretical evidence demonstrates that periodic twisting pitch in protein amyloid fibrils arises from the competing electrostatic and torsional elastic energies stored along the fibrils contour length. Amyloid fibrils are highly important in processes such as Alheimer’s and Parkinson’s disease. Jozef Adamcik and Raffaele Mezzenga , Soft Matter, 2011, Advance Article, DOI: 10.1039/C1SM05382E

Read all for free until June 17.

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Recent Hot Articles:Embryonic cell differentiation, superoleophobicity and magnetic nanoparticle/surfactant films.

Read for free until June 15!

SuperOLEOphobicity of lotus leaves: The superhydrophobicity of lotus leaves is well known, however, the lotus leaf also exhibits superoleophobicty on its lower side. Researchers have fabricated Janus interface materials with superhydrophobicity on one side in air and superleophobicity on the other side in water.  Qunfeng Cheng, Mingzhu Li, Yongmei Zheng, Bin Su, Shutao Wang and Lei Jiang, Soft Matter, 2011, Advance Article, DOI: 10.1039/C1SM05452J

 

Hydrogel scaffolds for embryonic cell differentiation: Embryonic cell (EC) differentiation can be strongly influenced by the scaffolds used to support the initial embryoid bodies’ growth. Here, molecular hydrogels are used for the first time to control EC differentiation, and could specifically enhance the expression of a mesoderm marker, Flk1Hongjie Liu, Yuehan Hu, Huaimin Wang, Jingyu Wang, Deling Kong, Ling Wang, Lingyi Chen and Zhimou Yang, Soft Matter, 2011, Advance Article, DOI: 10.1039/C1SM05198A

Towards synthesis of 2-D arrays of magnetic nanoparticles: The effect of magnetic nanoparticles on the surface stability of surfactant films, which is very important for a number of technical applications, has been investigated. Maghemite nanoparticles were added to SDS, CTAB and Brij 35 films, and the interactions studied using surface dilation and shear rheological experiments.

Patrick Degen, D. C. Florian Wieland, Sabine Leick, Michael Paulus, Heinz Rehage and Metin Tolan, Soft Matter, 2011, Advance Article, DOI: 10.1039/C1SM05248A.

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Recent Hot Articles: Creep compliance, uniform vesicles and polymer nanocapsule formation

Read these Hot Articles for free until June 12:

The ability to tune size and content of microscopic vesicles is hugely important for the chemical, food and biomedical industries. Continuous droplet interface crossing encapsulation (cDICE) was used to develop a high throughput low cost method for the synthesis of monodisperse vesicle suspensions in minutes. Droplets are continuously dripped from a capillary and simultaneously forced through an interface using centrifugal force.  Manouk Abkarian, Etienne Loiseau and Gladys Massiera, Soft Matter, 2011, 7, 4610-4614, DOI: 10.1039/C1SM05239J

The dilational flow properties of polymer films are studied using creep compliance. Mobile polymers constrained to a quasi-2D conformation have different flow properties to those of the bulk system. The compliance curves allowed calculation of the viscoelastic parameters and relaxation times of the Langmuir films, fully consistent with previous rheology experiments.
Hani Hilles and Francisco Monroy, Soft Matter, 2011, Advance Article, DOI: 10.1039/C1SM05255A

Polymer vesicles to hollow nanocapsules:
Head-tail type polycation self-assemblies of polyamidoamine dendron and poly(L-lysine) were cross-linked to form hollow nanocapsules. The nanocapsules show unique pH dependent properties, such as change in size and swelling.

Atsushi Harada, Shin-ichi Ichimura, Eiji Yuba and Kenji Kono, Soft Matter, 2011, 7, 4629-4635, DOI: 10.1039/C1SM05075C

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How do thin polymer films behave?

Rupture of thin polymer films

Molecular dynamics at nanometric length scales – Friedrich Kremer

Glassy dynamics and the glass transition temperature, in thin polymer films, has been a hotly debated area of polymer research over the last few decades. In particular, how and whether the glass transition temperature (Tg) changes from bulk values as the film thickness decreases or the molecular weight of the polymer is varied. A survey of the literature does little to clear this up with evidence being easily found for a lowering, a rise or no change in Tg as the film thickness is decreased. In his talk at the APS March meeting in Dallas, Friedrich Kremer discussed some of his recent work on polymer dynamics, with the aim of resolving some of the seemingly conflicting results found in the literature.

Kremer and his group used broadband dielectric spectroscopy (BDS), spectroscopic ellipsometry, x-ray reflectometry and differential scanning calorimetry to study thin polymer films (5-250 nm) with varying molecular weights. He noted that for the BDS measurements two different sample geometries were used. In the first evaporated aluminum electrodes (~80 nm) below and above the film were used as the counter electrodes. This is the common method used for BDS measurements. In the second geometry silicon wafers were used at the electrodes, with insulating silica nanostructures serving as spacers. This technique avoids evaporation of metal onto the films, allowing thinner films to be probed. Information on this technique can be found in Rev. Sci. Instrum.

In Kremer’s opinion sample preparation is the key to the conflicting results found in the literature. As such he was keen to stress that for all measurements his group performed, the samples were prepared in the same manner: Films were spin coated and then annealed at T = Tg + 50°C for 24hrs under vacuum in an oil-free environment. His results show that for films down to 5 nm in thickness and molecular weights varying from 300 – 8000 kg/mol, no change in the glassy dynamics is observed. No discrepancy between the different experimental techniques is seen, which all indicate that there is no shift in the glass transition temperature for thin films.

Kremer then went on to try to answer the questions of why he did not observe any change in the glassy dynamics while others do. The answer, he claims, can be found by carefully reading the experimental section of each paper. It all lies in how the films are prepared. Residual solvent can act as a plasticiser, while non-equilibrated films may have metastable states. Both of these result in altered dynamics. In the seminal paper on the polymer glass transition temperature by Keddie, Jones and Cory, they observe a decrease in of ~ 30°C in Tg as films thickness is reduced from 100 nm to 10nm. All films were annealed at 160°C for 48 hrs in vacuum – a temperature and time sufficient to remove residual solvent and relax stresses. However, experiments were conducted in air. According to Kremer, for PS at 150°C in air, chemical degradation occurs changing the molecular dynamics.

In other works, such as that of Dalnoki-Veress et al. and Ellison et al., annealing of the films after spin coating was insufficient to remove residual solvent, which would explain the molecular weight dependent behaviour and the altered dynamics observed. It should be mentioned that Kremer was keen to emphasise that he was not attacking any of these publications or their authors. He merely wanted to explain the reason for the divergence in results and show that films can be formed with no Tg shift. The results of Kremer and has group have been published in Macromolecules EPJ Special topics and Macromolecules.

Kremer is not the only one to have recently shown that sample preparation is key when considering the properties of thin polymer films. The papers by Raegen et al. and Thomas et al. agree with his conclusions and show that if films are annealed for long enough above Tg, then any anomalous behaviour disappears and bulk dynamics are restored.

The glassy dynamics of thin polymer films is still a hot area of debate, but perhaps recent papers show that there is in fact no contradiction in the literature. It just comes down to sample preparation!

Related articles in Soft Matter

Rotella et al., Probing interfacial mobility profiles via the impact of nanoscopic confinement on the strength of the dynamic glass transition.

Boucher et al., Physical aging of polystyrene/gold nanocomposites and its relation to the calorimetric Tg depression.

Bäumchen et al., Can liquids slide? Linking stability and dynamics of thin liquid films to microscopic material properties.

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Soft Matter Communication highlighted by ACS Noteworthy Chemistry

Self-folding all-polymer thermoresponsive microcapsules: Partially biodegradable thermoresponsive self-folding capsules capable of controlled capture and release of cells have been created by a team based in Germany. Georgi Stoychev, Nikolay Puretskiy and Leonid Ionov demonstrated the principle using star-like patterned polycaprolactone-poly(N-isopropylacrylamide) bilayers, which reversibly encapsulate/release yeast cells in response to a temperature signal. (Soft Matter, 2011, 7, 3277-3279.)

 Graphical abstract: Self-folding all-polymer thermoresponsive microcapsules

Read the full article for free until 31st May.

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