Soft Matter Blog

The term active matter might refer to something quite extraordinary such as, for example, dark matter. But what if I told you that such a term is used to describe very mundane things, such as flocks of birds or schools of fish? Active matter systems are composed of self-propelled elements and characterised by complex behaviours. Recently, the interest in active soft matter systems has grown significantly, with bacterial swarms and self-propelled particles being the two main foci of work in the area.

In this publication the authors are particularly interested in the less explored case in which two active components – faster and slower particles – are mixed. This would correspond, for example, to a mixture of bacteria with different speeds; or, in the macroscopic world, a scenario where zebras are being hunted by lions. They carry out Brownian dynamics simulations adapted to active matter, to find out that a segregation into condensed and gas-like phases, each composed of both fast and slow particles, takes place in such systems. Moreover, the composition of the condensed phase is highly dependent on the difference of activity between the two components. When the difference in speed between fast and slow particles is significant, the core of the condensed phase is composed of slow particles and the fast particles remain in the outer shell. If the speed is similar, then fast particles can be found as well as slow particles in the core of the condensed phase. The insights presented in this work are valuable to understand further multicomponent active matter already present in Nature or to engineer new active systems and harness the potential of their complex behaviour for diverse applications.

Comments from the authors:

• Multicomponent active matter composed of mixtures of particles with distinct active driving forces remains largely unexplored
• Active matter mixtures of fast and slow self-propelled colloids (Active Brownian Particles), is a useful way to investigate what quantities (if any) equilibrate in an active system
• The behaviour of an active material can be tuned by the introduction of additional distinct active species
• For binary active/active mixtures, a concentration weighted average of the activities of each species controls motility-induced phase separation (MIPS)
• For slow/fast activity ratio near 0, and low fast particle activity, particle participation in the dense phase is significantly affected due to mixing
• For slow/fast activity ratio near 1, and high fast particle activity, each species participates in the dense phase as if it were a monodisperse system
• Theoretical approaches for multi-component active mixtures will provide progress towards an equation of state for active matter

Citation to the paper: Heterogeneous versus homogeneous crystal nucleation of hard spheres, Soft Matter, 2020, 16, 1967-978, DOI: 10.1039/c9sm01799b

To read the full article click here!

Did you know that Soft Matter has an Active Matter themed collection? Click here to check out more papers!

About the web writer

Dr Nacho Martin-Fabiani (@FabianiNacho) is a Vice-Chancellor’s Research Fellow at the Department of Materials, Loughborough University, UK.

Crystallization processes from a liquid are very present in our life, for example the freezing of water and formation of ice crystals. In such processes, building blocks suspended in a fluid arrange themselves to form a crystal. Espinosa et al. focus on the case where these building blocks are hard spherical particles.

In a real-life situation, the liquid will normally be confined within certain walls – e.g. inside a container or on top of a surface. Therefore, some of the particles will interact with the walls and might form a small crystal (or nucleus) at its surface – the so-called heterogeneous nucleation. This is more favourable than generating a crystal in the bulk of the liquid (homogeneous nucleation), as it reduces the surface area of the crystal and thus the energy required to form it. In this work, the authors study the competition between both nucleation mechanisms. They carry out molecular dynamics simulations to model heterogeneous nucleation processes and compare with existing homogeneous nucleation numerical data. This enables them to identify two regimes depending on the density of the system, and in each of them a different nucleation mechanism is prevalent. When the number of particles in the liquid is not too high (less than 53% in volume), heterogeneous nucleation prevails. But above that threshold homogeneous nucleation takes over.

This study not only provides further insights into the competition between nucleation mechanisms; the expected prevalence of heterogeneous nucleation in experimental conditions could be part of the explanation as to why experimental and modelling results in the field have often discrepancies.

Comments from the authors:
• Above a certain density, a fluid composed hard spheres arranged in a disordered fashion is less stable than a crystal where the spheres are packed as cannon balls.
• The first step of the transformation into the crystal is the emergence of crystal embryos (or nuclei) in the fluid.
• Crystal nuclei may appear in the fluid bulk (homogeneous nucleation) or on the walls of the cell containing the fluid (heterogeneous nucleation).
• Whereas walls are always present in experiments, simulations effectively eliminate them with a trick called “periodic boundary conditions” by which a sphere leaving the simulation box on one side enters from the opposite side like ghosts in the Pacman game.
• The simultaneous appearance of homogeneous and heterogeneous nuclei makes it difficult to measure homogeneous nucleation rates in experiments.
• The balance between homogeneous and heterogeneous nucleation depends on the size and the shape of the container and on the fluid density.
• Heterogeneous nucleation prevails in fluids where particles occupy less than ~ 54 per cent of the space in cells typically used in experiments.
• The dominance of heterogeneous nucleation could explain long-standing discrepancies between experimental measurements and simulation estimates of the homogeneous nucleation rate.
• A strategy based on coating the cells with spheres arranged in a fluid fashion could potentially eliminate heterogeneous nucleation in experiments.

Citation to the paper: Heterogeneous versus homogeneous crystal nucleation of hard spheres, Soft Matter, 2019, 15, 9625-9631, DOI: 10.1039/C9SM01142K
To read the full article click here!

About the web writer

Dr Ignacio Martin-Fabiani (@FabianiNacho) is a Vice-Chancellor’s Research Fellow at the Department of Materials, Loughborough University, UK.

Cancer metastasis is believed to be responsible for about 90% cancer mortality¹. Despite tremendous efforts that have been devoted to identify biochemical markers for the onset of metastasis, the early diagnosis of malignant tumor remains challenging. Apart from interacting via biochemical signals with their surroundings, cancer cells are also actively generating forces and remodelling their mechanical microenvironment to survive and thrive, or even worse, to colonize new territories. In a recent paper published in Soft Matter, Zhang and others from the Pennsylvania State University established a traction force threshold that can be potentially used as a biomechanical marker for the onset of metastatic-like dispersion of cancer cells.

Spatiotemporal evolution of traction and intercellular tension in HCT-8 cell colonies cultured on soft and stiff hydrogel.

Upon prolonged culture of HCT-8 cells (a colon carcinoma cell line) on hydrogels with various stiffness, they found that their malignant transformation was both substrate stiffness and colony size dependent. HCT-8 colonies grown on soft hydrogels (2.6 kPa) remained cohesive throughout the two-week culture period. However, when cultured on stiffer substrates (20.7 kPa and 47.1 kPa), cells at the periphery started to disperse from their mother colony at day 7, until completely dispersed into individual cells at day 14. And size matters too. Dispersions in smaller colonies occurred earlier comparing to the larger ones.

Using traction force microscopy (TFM), researchers conducted a thorough study to correlate the cell-generated traction force with substrate stiffness and colony size. They revealed a fascinating spatiotemporal evolution of cellular forces during colony dispersion: cells on the verge generates critically higher traction forces comparing to colony interiors, ready to evade, until the onset of colony dispersion, where their forces start to decrease before completely vanished. Based on this traction force threshold, they further constructed a phase diagram to help predict colony cohesive/dispersive behaviour. When the force generated by the cells at colony boundary is big enough, they are more likely to metastasize. Interestingly, their dispersion behaviour could be efficiently suppressed by inhibiting their contractility, if treated at the right time.

The molecular mechanism behind remains to be further explored, but this study indicates the significant role of cell-generated forces in mediating cancer metastasis, which provides a new insight for the identification of early stage malignancy progression.

May the force not be with the cancer cells.

Reference:
¹ Chaffer, C. L., & Weinberg, R. A. (2011). Science, 331(6024), 1559-1564.

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A traction force threshold signifies metastatic phenotypic change in multicellular epithelia

About the web writer

Zhenwei Ma is currently a Ph.D. candidate in Mechanical Engineering at McGill University. He holds a M.E. degree in Chemical Engineering at McGill University and a B.E. degree in Chemical Engineering from Sichuan University. Find out more about him here.