Publication

To find out more, take a look at :

• the news published on the CNRS Biologie website,
• the scientific article published in Nature Microbiology.

• Read more about The secrets of Toxoplasma gondii parasite movement revealed

To find out more, take a look at :

• an explanatory video on the LIPhy Youtube channel,
• the news published on the CNRS Physique website,
• the scientific article published in Physical Review Fluids.

• Read more about When red blood cells go off the beaten track

The full article can be read here.

• Read more about Towards the ultimate precision limits: how information bounds estimation

Fish form schools in a wide variety of environments, from the open sea amid coral reefs to rocky rivers. Some natural environments are obstructed, it is therefore interesting to understand how well the collective structure of a shoal withstands the complexity of the environment. Scientists at LIPhy have demonstrated the existence of a behavioral transition when the environment of zebrafish becomes too crowded.

The study of collective movements provides a better understanding of the dynamic structures resulting from the self-organization of groups of individuals. The study of fish schools is part of this approach. The schooling of fish is remarkable because it occurs in a wide variety of environments, from the open sea to coral reefs. Previous research has focused on the study of schools in simple, unobstructed environments. In this work, a team from LIPhy used an approach combining experiment and modeling to study the impact of a complex environment on the collective organization of a school of fish. To do this, they followed the individual trajectories of a small group of zebrafish in the presence of variable obstacle densities. The structure of the school of fish proved quite resilient to the introduction of obstacles, maintaining an organization similar to that observed in the absence of obstacles. However, when the obstacles reach a critical density, the shoal structure disappears and the fish behave as if they were isolated, aligning themselves with the obstacles and no longer with their fellow fish. This density corresponds to a distance between obstacles close to the typical distance between fish in an undisturbed shoal, i.e. their natural social distance. Using a statistical model, the ingredients of this behavioral transition, from collective to independent, were analyzed in detail. The LIPhy team has shown that a complex environment can have a significant influence on the collective behavior of fish, and that social distance is critical for maintaining collective behavior in a complex environment. These results contribute to the emerging field of active and cognitive matter, and more broadly to the study of animal behavior and swarm robotics through biomimicry.

• Read more about Dissolution of a school of fish in a complex environment

The theoretical description of the rheology of dense suspensions of micrometric soft particles, in which thermal fluctuations are negligible, remains a very open field based mainly on direct numerical simulations of suspensions or on phenomenological macroscopic constitutive models. Starting from a simplified model of a two-dimensional dense suspension, researchers at LIPhy have proposed a statistical physics method to obtain a macroscopic constitutive model from the dynamics of the soft particles constituting the suspension.

The tensor constitutive model derived analytically in this way allows a continuous description of the suspensions in the same way as the already existing phenomenological models, while keeping a link with the scale of the constitutive entities. This constitutive model consists of a nonlinear evolution equation for the deviatoric part of the stress tensor. It allows to reproduce the existence of a threshold stress in the rheology of dense suspensions above the jamming density, and to describe a Bingham-type rheology at low shear rate. The tensor nature of the constitutive equation also allows the prediction of a threshold stress on the difference of the normal stresses.

The approach, which technically includes a number of approximations, is primarily aimed at obtaining a qualitative description of the phenomenology of dense suspensions of soft particles, and at relating the structure of the resulting equation to the ingredients of the microscopic dynamics of the suspension. Interestingly, the obtained constitutive equation also allows to address more complex and time-dependent protocols, such as the start or stop of shearing. In particular, relaxation after shear leads to a lower final stress for a stronger shear, in qualitative agreement with experiments. The model also reproduces the power-law decay of the shear rate in creep experiments, as well as the collapse of the storage modulus measured in parallel superposition protocols, in which a small sinusoidal strain is applied to a dense flowing suspension.

One of the main interests of this approach is that it allows one to link macroscopic rheology with microstructure. In particular, the approach predicts that the orientation of the anisotropy of the microstructure, which is governed by a competition between advection and contact elasticity, plays a key role in the flow properties.

Read more :

 Microscopic Theory for the Rheology of Jammed Soft Suspensions
Nicolas Cuny, Romain Mari, Eric Bertin
Physical Review Letters, Novembre 2021

 Dynamics of microstructure anisotropy and rheology of soft jammed suspensions
Nicolas Cuny, Eric Bertin, Romain Mari
Soft Matter, Janvier 2022

• Read more about Relating rheology to microstructure dynamics in dense suspensions of soft particles

Being able to activate and control microloads that would move within the bloodstream is a Grail of biomedical research. The current proposals are unfortunately characterized either by high technical complexity or by slow mobility, limited maneuverability and poor biocompatibility. Now, in this range of characteristic size around ten microns, an interesting candidate exists in the form of microbubbles covered with lipids, namely, already used for years as ultrasound contrast agents. Subjected to an ultrasonic’ pulse, they act as echogenic agents and allow a better visualization of vascularization, with resolutions improving from’year to year. For pulses of greater intensity,they can be destroyed and locally create stresses that open pathways between cells lining blood vessels, thereby promoting drug penetration towards their target.

In partnership with the University of Freiberg and the University of Twente, researchers and researchers from the Interdisciplinary Laboratory of Physics in Grenoble (LIPhy, CNRS /University Grenoble Alpes) studied the possibility of’activating these same microbubbles according to other acoustic modalities and demonstrated, via numerical simulations conducted in parallel with an experimental study, that, that these microbubbles can achieve a significant net displacement thanks to reproducible and non-destructive deflation and reinflation cycles. The direction of the swim can be controlled independently of the axis of propagation of the ultrasound, making these microbubbles good candidates for controlled piloting in ultrasound molecular imaging and drug delivery applications. Numerical modeling showed that well-designed microbubbles could swim at speeds of the order of m/s’ (an extraordinary order of magnitude given the size of these bubbles),thus allowing effective movements within the blood circulation. These results are published in the journal Engineering Communications.

References

Coated microbubbles swim via shell buckling, Georges Chabouh, Marcel Mokbel, Benjamin van Elburg, Michel Versluis, Tim Segers, Sebastian Aland, Catherine Quilliet, Gwennou Coupier, Communications Engineering, publié le 7 septembre 2023. Doi : 10.1038/s44172-023-00113-z

• Read more about Ultrasound helps microbubbles become swimming champions

For such big and fast objects, inertia combines with symmetry breaking (wing shape profile or ball rotation) to give rise to lift. However, lift forces are also at play at low Reynolds numbers, i.e. for small objects or slow flows where fluid viscosity dominates over inertia. Such forces stem from the key role played by the flow boundaries and the deformability of the objects involved: velocity gradients, elastic deformations or transport in boundary layers can lead to the emergence of lift forces. These are crucial in many soft matter and biophysics problems such as flows of suspensions, particle sorting, joint lubrication or blood circulation. In this article, we review three important mechanisms that give rise to lift and have initially been studied by separate research communities: (i) soft lubrication occurring when an object flows in the close vicinity of a deformable wall, (ii) elastohydrodynamic effects taking place when a deformable object is placed in a flow gradient, and (iii) electrokinetic lift arising from the transport of ions at the surface of an electrically charged object. We describe the main ingredients at the origin of such lift forces, discuss their respective magnitude and relevance, and point to other possible yet unexplored means of generating lift forces at zero Reynolds.

This review has been published in E. P. J. E. (https://link.springer.com/article/10.1140/epje/s10189-023-00369-5)

• Read more about Review : lift at low Reynolds number

Indeed, its geometry resembles that of the Tokamak used to magnetically confine hot plasmas for nuclear fusion experiments. 
When this bubble tokamak is excited by sound, the bubbles oscillate strongly. A first collective resonance of the 24 ring bubbles occurs, at around 500 Hz, twice as low as the resonance frequency of a single bubble, as the bubbles interact collectively in phase. At higher frequencies, other modes appear where the bubbles are not in phase.
An original feature of the acoustic field within the resonant Tokamak is its homogeneity, whereas acoustic fields usually vary greatly when moving close to a source, making it a unique acoustic object.

References:

Acoustic Tokamak with strongly coupled toroidal bubbles, A. Caumont, O. Stephan, E. Bossy, B. Dollet, C. Quilliet, and P. Marmottant, Physical Review E 108,  045105

• Read more about An acoustic "Tokamak"

Easy to collect and store, blood is the subject of numerous in vitro studies. Being made up of a large quantity of deformable cells, mainly red blood cells, it is characterized by a complex mechanical behavior, which impacts its various functions such as the supply of nutrients and oxygen to the organs, the elimination of waste, the regulation of body temperature and active immune surveillance. In France, several research teams have focused on the behavior of blood in the microcirculation, where red blood cells pass through vessels that are barely larger than themselves. But before being used in in vitro experiments, red blood cells are stored and manipulated under conditions that can impact their mechanical properties. However, there is no universal protocol and each research team follows its own empirical methods. Seven laboratories of the Mechanics of Materials and Biological Fluids Research Group  (GDR MÉCABIO¹) gathered their researchers around a table to share, compare and test their protocols.

The scientists were thus able to issue new recommendations to minimize the impact of blood storage and preparation conditions on the individual or collective movement of red blood cells under a wide range of flow conditions. Their response must be as close as possible to that expected in a physiological situation. This first step towards a standardization of practices should facilitate the comparison of the mechanical properties of red blood cells between different research teams. This will be of particular benefit to studies on diseases where the mechanical properties of blood are at stake, such as sickle cell disease. These results were published in the Biophysical Journal.

References:

Influence of storage and buffer composition on the mechanical behavior of flowing red blood cells. Adlan Merlo, Sylvain Losserand, François Yaya, Philippe Connes, Magalie Faivre, Sylvie Lorthois, Christophe Minetti, Elie Nader, Thomas Podgorski, Celine Renoux, Gwennou Coupier, and Emilie Franceschini.
Biophysical Journal, Volume 122, Issue 2, 17 January 2023.
https://doi.org/10.1016/j.bpj.2022.12.005

• Read more about Influence of storage and buffer composition on the mechanical behavior of flowing red blood cells

The development of fiber optic endoscopes is motivated by certain biomedical applications such as brain imaging. Multimode fibers, whose size is typically comparable to that of a human hair, are excellent candidates for minimizing the invasiveness of these procedures. However, the propagation of light in multimode fibers is very complex. Indeed, in this type of fiber, the light propagates in an unpredictable way, producing patterns that are difficult to interpret at the fiber outlet. Techniques currently exist to reconstruct images from such patterns, but these techniques are very sensitive to the deformations applied to the fiber, which strongly limits the possibilities of applications in biology.

In this study, LIPhy scientists demonstrate that it is possible to produce intelligible images with multimode fibers even when these are deformed. For this, they were inspired by the kaleidoscope of Sir David Brewster, and swapped the usual fibers, which have a circular heart, for fibers with a square heart. Indeed, the symmetry properties of these fibers generate a remarkable kaleidoscopic effect, which transports information in a robust way to deformations. They tested this method by reconstructing images of fluorescent micro-sources through the fiber, even as it was randomly distorted, thus reproducing the dynamic aspect of the disturbances that typically occur when studying living organisms. They thus demonstrated that, despite these disturbances, it is possible to reconstruct faithful images of these micro-sources through the fiber. This new technique thus promises to be promising for the development of miniature endoscopes for biomedical imaging.

References :

Speckle-correlation imaging through a kaleidoscopic multimode fiber
Dorian Bouchet, Antonio Miguel Caravaca-Aguirre, Guillaume Godefroy, Philippe Moreau, Irène Wang, Emmanuel Bossy. PNAS - Juin 2023. DOI

• Read more about Speckle correlation imaging through kaleidoscopic multimode fiber