Ripples in Thin Films
Seminar by Mazi Jalaal
from the Physics of Fluids laboratory
at the University of Twente, the Netherlands, EU
We present experimental observations of capillary ripples at the contact line of a droplet, spreading on a pre-wetted surface.
We use Digital Holographic Microscopy to measure the micro-scale undulation of the thin film. By raising the capillary number, the amplitude of the undulations increases at first and subsequently decreases.
At critical values of the capillary number, the ripples disappear. Using linear stability analysis, we further provide theoretical counterparts for the experimental observations, explaining the non-monotonic dependency on the capillary number
Place: PJ Lecture Hall Time: 9 October, 2018, 11:00
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.
Jalpa Soni reports on her outreach experience on 28 September 2018 to a local high school within the “European Researchers’ Night”.
On Spetember 28, 2018 under the realm of “European Researchers’ Night”, organised by Marie-Skolodwska-Curie Actions (MSCA, H2020), Brussels became the hub of science and research communication with the general public. Researchers from across Europe, mainly funded by various H2020 programs, gathered in Brussels to celebrate scientific temperament and spread its importance in everyday life.
Other than Brussels, universities and research institutions in over 340 cities all over Europe and neighbouring countries also participated in similar events where science and research was celebrated.
The University of Gothenburg (UGOT) also participated in this event by organising school visits for researchers to talk about science and life as a researcher to young students. Thanks to UGOT, I also got a chance to get involved in the “Researchers’ Friday” to go to a school and interact with students about my work and about researchers in general.
I visited the school named KLARA Teoretiska Gymnasium to talk to final year high school students who are about to enter university in a couple of months. Therefore, this was the ideal age group who might be interested in choosing science for higher education and would be curious about how is it to be a scientist.
I intended to tell them about my research project as well as to connect its implications in everyday scenarios of life. Beyond that, I was hoping for an engaging question-answer session where they could ask me anything related to science as a career.
I prepared a small speech where I could tell them about what I work on, and why, and to mention several related phenomena of nature. I also intended to tell them about the kind of applications of my experiments.
It was a wonderful experience. It really exceeded my expectations.
I have been involved in outreach activities before as well, but this was my first such experience in Sweden and I loved it. The students were very interested in what I had to say and what I was working in.
The following Q&A session was quite interesting as they asked many questions ranging from why I decided to study physics to how is it to live in different countries! Some wanted to know how scientists find the problems they work on and some were more interested in how do researchers keep motivated if an experiment fails!
At the end, they also had fun with the hands-on experiment I had brought with me to demonstrate some of the things I had talked about.
It was quite amazing to see that young students, on the verge of entering university, were so aware of the need of scientific mindset in general. I hope that some of them will choose research as their future interests and will contribute to the quest of knowledge.
Here is my speech:
My name is Jalpa and I am a researcher at the university of Gothenburg. I work at the department of Physics, which means I am a physicist. But what is it that I actually do? and more importantly why? Well, physics is behind almost everything we do in our everyday life! All of the technologies, radio, TV, computers, phones or the way we travel around with bikes, cars or planes came into existence because of physics. The very nature of universe can be understood with the laws of Physics. You might have heard the saying that “mathematics is the language of science”, and it’s true, isn’t it? But physics is the heart of science! From looking at stars in the universe to how we “see” things can be understood with Physics. However, today our knowledge has expanded so much that science is branched out in so many fields and subfields. And all these subfields are also being updated everyday, bringing more data. More data means more understanding. More data also means more challenges… and that means more technological developments. One of the recent example is the new iPhones that Apple announced this month. If you have followed, you might already know that their newer models are running on a nanometer size chip – that is one-billionth of a meter – claimed to be the smallest chip for a smartphone ever! That has been possible because physicists have been studying what happens at those scales with matter.
A billionth of a meter! Being able to study something that small is fascinating, right? A few decades ago, that would have been unimaginable, except for science fiction maybe. But today we talk about nanorobots that can go in our bodies and perform medical tasks for us! Technological advances have once again reduced the boundaries inside science and once again interdisciplinary science is becoming more exciting, to use it to improve life in general.
I also work in both biology and physics, occasionally using some chemistry as well as a bit of maths to explain the theory of my experiments. Among my various projects, the main theme is to study small things – of micron size – that is one-millionth of a meter. Specifically, I study the pattern of microorganisms (like bacteria), how they move around in various conditions.
But the effects I study with them are observed even in human scales. (showing some slides with images at this point)
For example, look at these penguins! These are emperor penguins, they live in Antarctica. These penguins huddle, gather around and move in large groups. And since it’s very cold environment where they live, they need to keep themselves warm! Look at these nice patterns they create while they move. They lean on the one in front of them and then rotate around in small steps, shifting positions from the outer side to the centre of the circle. This way, they are warm once inside the centre, the newcomers come and join the outside, but eventually everyone gets a chance to move inside for a while at least. The shifting pattern allows that to happen and everybody is happy.
Now look at these birds! They make beautiful patterns when they fly around together. As you might already guess, generally migrating birds make such large groups because it’s easier to keep the predators away. Also, it’s easier to hunt this way. In Denmark, they cause the effect of the “black sun” or the “sort sol” as the Danes call it. Every year in spring and autumn, the European starlings migrate from southern Europe to Scandinavia – near baltic sea – to breed. In Denmark, groups can get as big as a million birds and they cover the sun right around sunset to choose their nesting place, causing the “sort sol”.
In the ocean, large groups of fish also move in beautiful patterns!
Okay, all these patterns are nice to look at! but why are they important? right?
Well, look at this. Any of you find it familiar? It’s a scene of a crowd from one of the games, right? Did you know this particular game became a really big deal because of this particular scene? any guesses why? I will give you a hint – it’s the people! The number of people they simulated for this scene is what made the history. Wonder why is a crowd scene in a video game such a big deal? It’s because simulating a crowd of people is a lot more difficult than one would think! Look at this crowd simulation, you can see how it describes the people in a real crowd! There will be much more collisions and much more mingling in a real crowd! And it’s important to make crowd simulations more realistic to improve disaster management, isn’t it? For example, to design proper evacuation protocols in a fire-alarm situations, or for earthquake evacuation protocols. It would be good to be able to design public places accommodating good emergency protocols! Understanding these patterns of nature can help us achieve that on a more efficient manner.
And now let’s get back to the small world! At micron scales, look at these bacteria – they behave in nice and familiar looking patterns as well! And it’s important to understand how they move in various environment, like how they spread on a bad slice of bread, or in a rotten fruit, or in our body! Such studies could tell us how to stop the unwanted ones to enter our system and to select the good ones for benefits. Because not all bacteria are bad, some are good for our body, help us digest our food for example.
So, one of my project is related to this. We study bacteria in a complex environment and see how they find their way around it. We put some bacteria and some small particles (around the same size as bacteria but made of silica) together and monitored what happens to the bacteria. As it turns out they make highways, which are reused by the following bacteria, and this way they actually move in longer distances compared to when there are no obstacles. Bacteria alone move in more circular patterns, while in an obstacle environment their circles get bigger! We are trying to understand the mechanism behind this kind of motion and we want to see if that can be used to design artificial robots based on bacterial motion.
Now, I also want to study these things in three dimensions, more realistic! The read world is 3D! So I am building a microscope to do 3D imaging at high speeds to monitor live motions of these microorganisms. It’s called a light-sheet microscope and it looks like this! Not at all like a typical microscope! And this is one of the 3D video I took earlier this week. It’s short, but I think you can see the 3D volume and the motion of particles in 3D.
So, this is what I do! I also work with some other projects and I will talk about them if you are interested. Thank you for listening and feel free to ask any questions!
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 . A number of single-particle and collective phenomena in active matter will be adressed ranging from the circle swimming to inertial delay effects.
 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).
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.
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.
Intracavity Optical Trapping
Fatemeh Kalantarifard, Parviz Elahi, Ghaith Makey, Onofrio M. Maragò, F. Ömer Ilday & Giovanni Volpe
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.
High-Performance Reconstruction of Microscopic Force Fields from Brownian Trajectories Laura Pérez García, Jaime Donlucas Pérez, Giorgio Volpe, Alejandro V. Arzola & Giovanni Volpe
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 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)
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
Maximilian Leyman, Freddie Ogemark, Jan Wehr & Giovanni Volpe
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.