2020 Impact factor 4.590
Particles and Fields

EPJ E Highlight - Simulating microswimmers in nematic fluids

Microswimmer pushes through nematic liquid crystal

A combination of two simulation techniques has allowed researchers to investigate how swimming microparticles propel themselves through ‘nematic liquid crystals’ – revealing some unusual behaviours

Artificial microswimmers have received much attention in recent years. By mimicking microbes which convert their surrounding energy into swimming motions, these particles could soon be exploited for many important applications. Yet before this can happen, researchers must develop methods to better control the trajectories of individual microswimmers in complex environments. In a new study published in EPJ E, Shubhadeep Mandal at the Indian Institute of Technology Guwahati (India), and Marco Mazza at the Max Planck Institute for Dynamics and Self-Organisation in Göttingen (Germany) and Loughborough University (UK), show how this control could be achieved using exotic materials named ‘nematic liquid crystals’ (LCs) – whose viscosity and elasticity can vary depending on the direction of an applied force.

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EPJ E Highlight - Micro-environmental influences on artificial micromotors

Janus particles propel themselves forward

New experiments reveal the characteristic ways in which self-propelled ‘Janus particles’ with charged coatings will slide across or move away from charged boundaries in their surrounding environments.

By harvesting energy from their surrounding environments, particles named ‘artificial micromotors’ can propel themselves in specific directions when placed in aqueous solutions. In current research, a popular choice of micromotor is the spherical ‘Janus particle’ – featuring two distinct sides with different physical properties. Until now, however, few studies have explored how these particles interact with other objects in their surrounding microenvironments. In an experiment detailed in EPJ E, researchers in Germany and The Netherlands, led by Larysa Baraban at Helmholtz-Zentrum Dresden-Rossendorf, show for the first time how the velocities of Janus particles relate to the physical properties of nearby barriers.

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EPJ E Highlight - Modelling speed-ups in nutrient-seeking bacteria

Running and tumbling towards nutrient sources

By considering how some bacteria will swim faster within higher nutrient concentrations, researchers have created a more accurate model of how these microbes search for nutrients

Many bacteria swim towards nutrients by rotating the helix-shaped flagella attached to their bodies. As they move, the cells can either ‘run’ in a straight line, or ‘tumble’ by varying the rotational directions of their flagella, causing their paths to randomly change course. Through a process named ‘chemotaxis,’ bacteria can decrease their rate of tumbling at higher concentrations of nutrients, while maintaining their swimming speeds. In more hospitable environments like the gut, this helps them to seek out nutrients more easily. However, in more nutrient-sparse environments, some species of bacteria will also perform ‘chemokinesis’: increasing their swim speeds as nutrient concentrations increase, without changing their tumbling rates. Through new research published in EPJ E, Theresa Jakuszeit and a team at the University of Cambridge led by Ottavio Croze produced a model which accurately accounts for the combined influences of these two motions.

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EPJ E Highlight - Using neutron scattering to better understand milk composition

Simulated x-ray scattering from casein micelles in normal milk concentrations compare a new model with existing results.

By using a more complex model for neutron scattering data, researchers can better understand the composition of materials such as milk.

Neutron scattering is a technique commonly used in physics and biology to understand the composition of complex multicomponent mixtures and is increasingly being used to study applied materials such as food. A new paper published in EPJ E by Gregory N Smith, Niels Bohr Institute, University of Copenhagen, Denmark, shows an example of neutron scattering in the area of food science. Smith uses neutron scattering to better investigate casein micelles in milk, with the aim of developing an approach for future research.

Smith, also a researcher at the ISIS Neutron and Muon Source in the UK, explains why better modelling of how neutrons are scattered by structures in colloid materials is important. “How well you can understand the structure of a system from scattering data depends on how good your model is, and the better and more realistic your model, the better your understanding,” the researcher says. “This is true for food as for any material. A better understanding of the structure of casein in milk can help better understand dairy products.”

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EPJ E Highlight - Trapping nanoparticles with optical tweezers

Trapping fluorescent particles with Arago spots.

By exploiting a particular property of light diffraction at the interface between a glass and a liquid, researchers have demonstrated the first optical tweezers capable of trapping nanoscale particles.

Optical tweezers are a rapidly growing technology, and have opened up a wide variety of research applications in recent years. The devices operate by trapping particles at the focal points of tightly focused laser beams, allowing researchers to manipulate the objects without any physical contact. So far, optical tweezers have been used to confine objects just micrometres across – yet there is now a growing desire amongst researchers to extend the technology to nanometre-scale particles. In new research published in EPJ E, Janine Emile and Olivier Emile at the University of Rennes, France, demonstrate a novel tweezer design, which enabled them to trap fluorescent particles just 200 nanometres across for the first time.

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EPJ E Highlight - Characterising complex flows in 2D bubble swarms

Bubble swarms introduce complex flows.

In 2D simulations, the flows surrounding rising swarms of bubbles display characteristically different behaviours to those observed in 3D models

When swarms of bubbles are driven upwards through a fluid by their buoyancy, they can generate complex flow patterns in their wake. Named ‘pseudo-turbulence,’ these patterns are characterised by a universal mathematical relationship between the energy of flows of different sizes, and the frequency of their occurrence. This relationship has now been widely observed through 3D simulations, but it is less clear whether it would still hold for 2D swarms of bubbles. Through research published in EPJ E, Rashmi Ramadugu and colleagues at the TIFR Centre for Interdisciplinary Sciences in Hyderabad, India, show that in 2D simulated fluids, this pattern changes within larger-scale flows in less viscous fluids.

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EPJ E Highlight - Building better diffusion models for active systems

Deviating from Brownian motion.

Modifications to existing theories have enabled researchers to better understand and model the dynamics of systems which don’t obey conventional laws of diffusion

In normal circumstances, particles will follow well-established random motions as they diffuse through liquids and gases. Yet in some types of system, this behaviour can be disrupted – meaning the diffusion motions of particles are no longer influenced by the outcomes of chains of previous events. Through research published in EPJ E, Bernhard Mitterwallner, a Ph.D. student in the team of Roland Netz at the Free University of Berlin, Germany, has developed new theories detailing how these unusual dynamics can be reproduced in generalised mathematical models.

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EPJ E Highlight - Modelling microswimmers for drug delivery

Streamlines of the fluid flow induced by a single point force (Stokeslet) representing a microswimmer inside a freely moving viscous drop.

Mathematical models of the motion of cells in viscous liquids that show how this motion is affected by the presence of a surfactant coating have applications in the design of artificial microswimmers for targeted drug delivery, micro-surgery and other applications.

Many types of motile cells, such as the bacteria in our guts and spermatozoa in the female reproductive tracts, need to propel themselves through confined spaces filled with viscous liquid. In recent years, the motion of these ‘microswimmers’ has been mimicked in the design of self-propelled micro- and nano-scale machines for applications including targeted drug delivery. Optimising the design of these machines requires a detailed, mathematical understanding of microswimmers in these environments. A large, international group of physicists led by Abdallah Daddi-Moussa-Ider of Heinrich-Heine-Universität Düsseldorf, Germany has now generated mathematical models of microswimmers in clean and surfactant-covered viscous drops, showing that the surfactant significantly alters the swimmers’ behaviour. They have published their work in EPJ E.

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EPJ E Celebrates 20th Anniversary

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In 2020 The European Physical Journal E - Soft Matter and Biological Physics marks twenty years since the publication of its first papers. To commemorate this milestone, the Editors-in-Chief have made a selection of articles published over these two decades, representing the breadth of coverage of the journal. These articles are now freely accessible until 15 October via this Q&A with Prof Holger Stark, Editor-in-Chief of EPJ E.

EPJ E Highlight - Polymers can fine-tune attractions between suspended nanocubes

Simulating motions of bottom-heavy squirmers.

Interactions between hollow silica nanocubes suspended in a solution can be adjusted by varying the concentration of polymer molecules added to the mixture.

Colloids are complex mixtures in which microscopic particles of one substance are suspended evenly throughout another. They can be prepared in many different ways, but to achieve desirable properties in the final mixture, researchers must maintain a delicate control over the interactions which take place between the particles. In new research published in EPJ E, a team led by Remco Tuinier at the Eindhoven University of Technology in the Netherlands demonstrate this level of control for a type of colloid in which the suspended particles take the form of hollow, nanoscale cubes – a case which has only previously been explored through theoretical calculations.

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Editors-in-Chief
G. Dissertori, J. Monroe, K. Skenderis and D. Zeppenfeld

Deputy Editors-in-Chief
D. J. Schwarz

Thank you for the excellent service of EPJC during submission.

Anto Sulaksono, Universitas Indonesia, Depok, Indonesia

ISSN: 1434-6052 (Electronic Edition)

© Società Italiana di Fisica and
Springer-Verlag