News

Seminar on cell differentiation by Mariana Benitez Keinrad from UNAM, Soliden 3rd floor, 8 oct 18

Cell differentiation and pattern formation in the transition to multicellularity: lessons from the microbial world
Seminar by Mariana Benitez Keinrad
from the Laboratorio Nacional de Ciencias de la Sostenibilidad,
Universidad Nacional Autónoma de México (UNAM), Mexico.

Multicellular development occurs in plants, animals and other lineages, and involves the complex interaction among biochemical, physical and ecological factors. Our group has focused on the study of microbial multicellular organisms, which have been considered useful models to study the evolutionary transition to multicelullarity. I present some of our theoretical and experimental work, and discuss the physical and chemical processes that, in coordination with molecular regulatory networks, appear to be relevant for cell differentiation, patterning and morphogenesis in microbial aggregates.

Place: Soliden 3rd floor
Time: 8 October, 2018, 12:15

Alessandro Magazzù awarded Best Presentation Prize at Soft Matter Days 2018

Alessandro Magazzù has been awarded a best oral contribution “Soft Matter poster price” during the conference Italian Soft Matter Days 2018, held in Padua, Italy on September 13-14, 2018. The prize has been given by Emanuela Zaccarelli, editorial board members of the Soft Matter journal. This prize mainly consists in an invitation to submit a manuscript without the pre-screening by the Editors. It also includes a “poster prize” and a personal yearly subscription to the journal.

Colloquium on active matter by Hartmut Löwen, PJ Lecture Hall, 13 sep 18

Physics of active soft matter
General Physics Colloquium by Hartmut Löwen, Heinrich-Heine Universität Düsseldorf​, Germany

​Abstract: Ordinary materials are “passive” in the sense that their constituents are typically made by inert particles which are subjected to thermal fluctuations, internal interactions and external fields but do not move on their own. Living systems, like schools of fish, swarms of birds, pedestrians and swimming microbes are called “active matter” since they are composed of self-propelled constituents. Active matter is intrinsically in nonequilibrium and exhibits a plethora of novel phenomena as revealed by a recent combined effort
of statistical theory, computer simulation and real-space experiments. The colloquium talk provides an introduction into the physics of active matter focussing on biological and artificial microswimmers as key examples of active soft matter [1]. A number of single-particle and collective phenomena in active matter will be adressed ranging from the circle swimming to inertial delay effects.​​

​[1] For a review, see: C. Bechinger, R. di Leonardo, H. Löwen, C. Reichhardt, G. Volpe, G. Volpe, Active particles in complex and crowded environments, Reviews of Modern Physics 88, 045006 (2016).

Place: PJ Lecture Hall

Alejandro V. Arzola visits the Soft Matter Lab. Welcome!

Alejandro V. Arzola is a Visiting Professor from the Universidad Nacional Autónoma de México in Mexico City. His visiting position is financed through the Linnaeus Palme International Exchange Programme.

Alejandro was born in Oaxaca in the south of Mexico. He studied for a PhD at the Universidad Nacional Autónoma de México (UNAM) in Mexico City, worked as a posdoctoral researcher at the Institutte of Scientific Instruments in Brno, Czech Republic, and at UNAM. Since 2014 he joined the group of Optical Micromanipulation at the Institute of Physics in UNAM.

He is interested in optical micromanipulation and related research fields. His latest research deals with the transport of Brownian particles in optical landscapes under breaking space-time symmetries, a system which is known in the literature as ratchets. He is also interested in the behavior of microscopic particles in structured light fields with spin and orbital angular momentum.

Talk by G. Volpe at SPIE OTOM XV, San Diego, 23 Aug 18

Microscopic Engine Powered by Critical Demixing
Falko Schmidt, Alessandro Magazzù, Agnese Callegari, Luca Biancofiore, Frank Cichos & Giovanni Volpe
SPIE Nanoscience + Engineering, Optical trapping and Optical Manipulation XV, San Diego (CA), USA
19-23 August 2018

During the last few decades much effort has gone into the miniaturization of machines down to the microscopic scale with robotic solutions indispensable in modern industrial processes and play a central role in many biological systems. There has been a quest in understanding the mechanism behind molecular motors and several approaches have been proposed to realize artificial engines capable of converting energy into mechanical work. These current micronsized engines depend on the transfer of angular momentum of light, are driven by external magnetic fields, due to chemical reactions or by the energy flow between two thermal reservoirs. Here we propose a new type of engine that is powered by the local, reversible demixing of a critical binary liquid. In particular, we show that an absorbing, optically trapped particle performs revolutions around the optical beam because of the emergence of diffusiophoresis and thereby produces work. This engines is adjustable by the optical power supplied, the temperature of the environment and the criticality of the system.

Reference: Schmidt et al., Phys. Rev. Lett. 120(6), 068004 (2018) DOI: 10.1103/PhysRevLett.120.068004

Intracavity Optical Trapping preprint on ArXiv

Intracavity Optical Trapping

Intracavity Optical Trapping
Fatemeh Kalantarifard, Parviz Elahi, Ghaith Makey, Onofrio M. Maragò, F. Ömer Ilday & Giovanni Volpe
arXiv: 1808.07831

Standard optical tweezers rely on optical forces that arise when a focused laser beam interacts with a microscopic particle: scattering forces, which push the particle along the beam direction, and gradient forces, which attract it towards the high-intensity focal spot. Importantly, the incoming laser beam is not affected by the particle position because the particle is outside the laser cavity. Here, we demonstrate that intracavity nonlinear feedback forces emerge when the particle is placed inside the optical cavity, resulting in orders-of-magnitude higher confinement along the three axes per unit laser intensity on the sample. We present a toy model that intuitively explains how the microparticle position and the laser power become nonlinearly coupled: The loss of the laser cavity depends on the particle position due to scattering, so the laser intensity grows whenever the particle tries to escape. This scheme allows trapping at very low numerical apertures and reduces the laser intensity to which the particle is exposed by two orders of magnitude compared to a standard 3D optical tweezers. We experimentally realize this concept by optically trapping microscopic polystyrene and silica particles inside the ring cavity of a fiber laser. These results are highly relevant for many applications requiring manipulation of samples that are subject to photodamage, such as in biological systems and nanosciences.

Force Reconstruction via Maximum-likelihood-estimator (MLE) Analysis (FORMA) preprint on ArXiv

High-Performance Reconstruction of Microscopic Force Fields from Brownian Trajectories

High-Performance Reconstruction of Microscopic Force Fields from Brownian Trajectories
Laura Pérez García, Jaime Donlucas Pérez, Giorgio Volpe, Alejandro V. Arzola & Giovanni Volpe
arXiv: 1808.05468

The accurate measurement of microscopic force fields is crucial in many branches of science and technology, from biophotonics and mechanobiology to microscopy and optomechanics. These forces are often probed by analysing their influence on the motion of Brownian particles. Here, we introduce a powerful algorithm for microscopic Force Reconstruction via Maximum-likelihood-estimator (MLE) Analysis (FORMA) to retrieve the force field acting on a Brownian particle from the analysis of its displacements. FORMA yields accurate simultaneous estimations of both the conservative and non-conservative components of the force field with important advantages over established techniques, being parameter-free, requiring ten-fold less data and executing orders-of- magnitude faster. We first demonstrate FORMA performance using optical tweezers. We then show how, outperforming any other available technique, FORMA can identify and characterise stable and unstable equilibrium points in generic extended force fields. Thanks to its high performance, this new algorithm can accelerate the development of microscopic and nanoscopic force transducers capable of operating with high reliability, speed, accuracy and precision for applications in physics, biology and engineering.

Stability of Brain Graph Measures published in Sci. Rep.

Stability of graph theoretical
measures in structural brain
networks in Alzheimer’s disease

Stability of graph theoretical measures in structural brain networks in Alzheimer’s disease
Gustav Mårtensson, Joana B. Pereira, Patrizia Mecocci, Bruno Vellas, Magda Tsolaki, Iwona Kłoszewska, Hilkka Soininen, Simon Lovestone, Andrew Simmons, Giovanni Volpe & Eric Westman
Scientific Reports 8, 11592 (2018)
DOI: 10.1038/s41598-018-29927-0

Graph analysis has become a popular approach to study structural brain networks in neurodegenerative disorders such as Alzheimer’s disease (AD). However, reported results across similar studies are often not consistent. In this paper we investigated the stability of the graph analysis measures clustering, path length, global efficiency and transitivity in a cohort of AD (N = 293) and control subjects (N = 293). More specifically, we studied the effect that group size and composition, choice of neuroanatomical atlas, and choice of cortical measure (thickness or volume) have on binary and weighted network properties and relate them to the magnitude of the differences between groups of AD and control subjects. Our results showed that specific group composition heavily influenced the network properties, particularly for groups with less than 150 subjects. Weighted measures generally required fewer subjects to stabilize and all assessed measures showed robust significant differences, consistent across atlases and cortical measures. However, all these measures were driven by the average correlation strength, which implies a limitation of capturing more complex features in weighted networks. In binary graphs, significant differences were only found in the global efficiency and transitivity measures when using cortical thickness measures to define edges. The findings were consistent across the two atlases, but no differences were found when using cortical volumes. Our findings merits future investigations of weighted brain networks and suggest that cortical thickness measures should be preferred in future AD studies if using binary networks. Further, studying cortical networks in small cohorts should be complemented by analyzing smaller, subsampled groups to reduce the risk that findings are spurious.

Phototactic Robot Tunable by Sensorial Delays preprint in arXiv

Phototactic Robot Tunable by Sensorial Delays

Phototactic Robot Tunable by Sensorial Delays
Maximilian Leyman, Freddie Ogemark, Jan Wehr & Giovanni Volpe
arXiv: 1807.11765

The presence of a delay between sensing and reacting to a signal can determine the long-term behavior of autonomous agents whose motion is intrinsically noisy.
In a previous work [M. Mijalkov, A. McDaniel, J. Wehr, and G. Volpe, Phys. Rev. X 6, 011008 (2016)], we have shown that sensorial delay can alter the drift and the position probability distribution of an autonomous agent whose speed depends on the illumination intensity it measures. Here, using theory, simulations, and experiments with a phototactic robot, we generalize this effect to an agent for which both speed and rotational diffusion depend on the illumination intensity and are subject to two independent sensorial delays. We show that both the drift and the probability distribution are influenced by the presence of these sensorial delays. In particular, the radial drift may have positive as well as negative sign, and the position probability distribution peaks in different regions depending on the delay.
Furthermore, the presence of multiple sensorial delays permits us to explore the role of the interaction between them.

Review on Optical Tweezers published in J. Quant. Spectrosc. Rad. Transf.

Optical tweezers and their applications

Optical tweezers and their applications
Paolo Polimeno, Alessandro Magazzù, Maria Antonia Iata, Francesco Patti, Rosalba  Saija, Cristian Degli Esposti Boschi, Maria Grazia Donato, Pietro G. Gucciardi, Philip H. Jones, Giovanni Volpe & Onofrio M. Maragò
Journal of Quantitative Spectroscopy and Radiative Transfer 218(October 2018), 131—150 (2018)
DOI: 10.1016/j.jqsrt.2018.07.013

Optical tweezers, tools based on strongly focused light, enable optical trapping, manipulation, and characterisation of a wide range of microscopic and nanoscopic materials. In the limiting cases of spherical particles either much smaller or much larger than the trapping wavelength, the force in optical tweezers separates into a conservative gradient force, which is proportional to the light intensity gradient and responsible for trapping, and a non-conservative scattering force, which is proportional to the light intensity and is generally detrimental for trapping, but fundamental for optical manipulation and laser cooling. For non-spherical particles or at intermediate (meso)scales, the situation is more complex and this traditional identification of gradient and scattering force is more elusive. Moreover, shape and composition can have dramatic consequences for optically trapped particle dynamics. Here, after an introduction to the theory and practice of optical forces with a focus on the role of shape and composition, we give an overview of some recent applications to biology, nanotechnology, spectroscopy, stochastic thermodynamics, critical Casimir forces, and active matter.