Category: Presentation

Poster by A. Callegari at SPIE-OTOM, San Diego, 21 August 2023

Schematic of the scattering of a light ray on a Janus particle. (Image by A. Callegari.)
Janus Particles in Geometrical Optics
Agnese Callegari, Giovanni Volpe
SPIE-OTOM, San Diego, CA, USA, 20 – 24 August 2023
Date: 21 August 2023

Janus particles are microscopic objects characterized by one feature with dual properties. Typical examples of Janus particles are metal-coated silica particles, widely used in soft and active matter applications because of their versatility and relative simplicity of their fabrication. Janus particles are often utilized in the presence of optical potentials. Given the non-homogeneous nature of their refractive index composition, the interaction between the Janus particle and light is non-trivial to model: in addition to the optical force, the particle experiences an optical torque, even in the case of spherically shaped Janus particles, and its metallic cap can also absorb part of the optical power impinging on the particle. Here, we provide a description of the Janus particle in the geometrical optics approximation, and an implementation for calculating forces, torques, and absorption on partially coated Janus particles of spherical and ellipsoidal shape. This implementation is based on the existing OTGO toolbox, developed in Matlab for calculating optical forces and torques in the geometrical optics regime. We first validate our model against the known experimental results and show that interesting dynamical effects arise in the presence of travelling-wave optical potential.

Presentation by A. Callegari at SPIE-OTOM, San Diego, 21 August 2023

One exemplar of the HEXBUGS used in the experiment. (Image by the Authors of the manuscript.)
Playing with Active Matter
Angelo Barona Balda, Aykut Argun, Agnese Callegari, Giovanni Volpe
SPIE-OTOM, San Diego, CA, USA, 20 – 24 August 2023
Date: 21 August 2023

In the last 20 years, active matter has been a very successful research field, bridging the fundamental physics of nonequilibrium thermodynamics with applications in robotics, biology, and medicine. This field deals with active particles, which, differently from passive Brownian particles, can harness energy to generate complex motions and emerging behaviors. Most active-matter experiments are performed with microscopic particles and require advanced microfabrication and microscopy techniques. Here, we propose some macroscopic experiments with active matter employing commercially available toy robots, i.e., the Hexbugs. We demonstrate how they can be easily modified to perform regular and chiral active Brownian motion. We also show that Hexbugs can interact with passive objects present in their environment and, depending on their shape, set them in motion and rotation. Furthermore, we show that, by introducing obstacles in the environment, we can sort the robots based on their motility and chirality. Finally, we demonstrate the emergence of Casimir-like activity-induced attraction between planar objects in the presence of active particles in the environment.

Reference
Angelo Barona Balda, Aykut Argun, Agnese Callegari, Giovanni Volpe, Playing with Active Matter, arXiv: 2209.04168

Soft Matter Lab members present at SPIE Optics+Photonics conference in San Diego, 20-24 August 2023

The Soft Matter Lab participates to the SPIE Optics+Photonics conference in San Diego, CA, USA, 20-24 August 2023, with the presentations listed below.

Giovanni Volpe is also co-author of the presentations:

  • Jiawei Sun (KI): (Poster) Assessment of nonlinear changes in functional brain connectivity during aging using deep learning
    21 August 2023 • 5:30 PM – 7:00 PM PDT | Conv. Ctr. Exhibit Hall A
  • Blanca Zufiria Gerbolés (KI): (Poster) Exploring age-related changes in anatomical brain connectivity using deep learning analysis in cognitively healthy individuals
    21 August 2023 • 5:30 PM – 7:00 PM PDT | Conv. Ctr. Exhibit Hall A
  • Mite Mijalkov (KI): Uncovering vulnerable connections in the aging brain using reservoir computing
    22 August 2023 • 9:15 AM – 9:30 AM PDT | Conv. Ctr. Room 6C

Presentation by M. Rey at UK COLLOIDS, Liverpool, 17 July 2023

Interfacial self-assembly behaviour of soft core-shell particles. (Image by M. Rey.)
Versatile strategy for homogeneous drying of dispersed particles
Marcel Rey,
UK COLLOIDS 2023
Date: 17 July 2023
Time: 11:20 (CET)

After spilling coffee, a tell-tale stain is left by the drying droplet. This universal phenomenon, known as the “coffee ring effect”, is observed independent of the dispersed material. However, for many technological processes such as coating techniques and ink-jet printing a uniform particle deposition is required and the coffee ring effect is a major drawback.
Here, we present a simple and versatile strategy to achieve homogeneous drying patterns by modifying the surface of the dispersed particles with surface-active polymers. A particle dispersion is mixed with excess surface-active polymers (e.g. polyvinyl alcohol). The polymer partially adsorbs onto the particles and excess polymer is removed by centrifugation and redispersion. While pure particle dispersions form a typical coffee ring, the polymer-modified dispersions dry into a uniform particle deposit. In this talk, I will discuss how the polymer coating prevents accumulation and pinning at the droplet edge and leads to a uniform particle deposition after drying.
It should be highlighted that the presented method is independent of particle shape (e.g. spherical, ellipsoidal or ill-defined particle shapes) and is applicable to a variety of commercial pigment particles (e.g. hematite, goethite or titanium dioxide). Further, the method works for different dispersion media (e.g. aqueous, polar and apolar solvents), demonstrating the practicality of this work for everyday processes.

Invited talk by M. Rey at the University of Manchester, 20 July, 2023

Interfacial self-assembly behaviour of soft core-shell particles. (Image by M. Rey.)
Complex self-assembly / Overcoming the coffee ring effect
Marcel Rey
Presentation for the School of Materials at the University of Manchester
Date: 20 July 2023

In this seminar, I will talk about complex self-assembly behaviour of simple building blocks. Afterwards, I will introduce a simple yet versatile strategy to overcome the coffee ring effect and obtain homogeneous drying of particle dispersions.

Spherical colloidal particles confined at liquid interfaces typically self-assemble into hexagonal packing. Here, I will show that much more complex self-assembly behaviour is possible spherical particles with a hard-core / soft-shell architecture. Upon compression, these core-shell particles transition from a hexagonal packing to a chain packing, then to a square packing and finally to a hexagonal close packing. I will rationalize these experimental observations with calculations and simulations using simple core-shell potentials.

After spilling coffee, a tell-tale circular stain is left by the drying droplet. This universal phenomenon, known as the “coffee ring effect”, is observed independent of the suspended material. We recently developed a simple yet versatile strategy to achieve homogeneous drying of dispersed particles. Modifying the particle surface with surface-active polymers provides enhanced steric stabilization and facilitates adsorption to the liquid/air interface which, after drying, leads to uniform particle deposition. This method is independent of particle size and shape and applicable to a variety of commercial pigment particles promising applications in daily life.

 

Poster presentation by M. Rey at DINAMO Svolvaer, Norway, 13 June 2023

Core-shell microgel in an optical tweezer. (Image by M. Rey.)
Optical characterisation of soft microgels
Marcel Rey,
DINAMO 2023
Date: 13 June 2023
Time: 19:00 (CET)

Soft microgels are ideal model systems due to their ability to deform and adapt their shape upon external stimuli. Here, we use optical tweezers to measure the diffusion of soft core-shell microgels. We report an anomalous, subdiffusive behaviour, which may be linked to the multiple length scales present within core-shell microgels.

Invited Talk by A. Callegari at PIERS 2023, Prague, 3 July 2023

An illustration of microscopic gold flakes on surface. (Image by F. Schmidt.)
Tunable critical Casimir forces counteract Casimir–Lifshitz attraction
Agnese Callegari
PIERS 2023, Prague, Czech Republic
3 July 2023, 09:40

Casimir forces in quantum electrodynamics emerge between microscopic metallic objects because of the confinement of the vacuum electromagnetic fluctuations occurring even at zero temperature. Their generalization at finite temperature and in material media are referred to as Casimir–Lifshitz forces. These forces are typically attractive, leading to the widespread problem of stiction between the metallic parts of micro- and nanodevices. Recently, repulsive Casimir forces have been experimentally realized but their reliance on specialized materials prevents their dynamic control and thus limits their further applicability. Here, we experimentally demonstrate that repulsive critical Casimir forces, which emerge in a critical binary liquid mixture upon approaching the critical temperature, can be used to actively control microscopic and nanoscopic objects with nanometer precision. We demonstrate this by using critical Casimir forces to prevent the stiction caused by the Casimir–Lifshitz forces. We study a microscopic gold flake above a flat gold-coated substrate immersed in a critical mixture. Far from the critical temperature, stiction occurs because of dominant Casimir–Lifshitz forces. Upon approaching the critical temperature, however, we observe the emergence of repulsive critical Casimir forces that are sufficiently strong to counteract stiction. Our method provides a novel way of controlling the distances of micro- and nanostructures using tunable critical Casimir forces to counteract forces such as the Casimir–Lifshitz force, thereby preventing stiction and device failure. Due to the simplicity of our design the concept can be adapted to already existing MEMS and NEMS. Moreover, this path opens new possibilities for the dynamic control of MEMS and NEMS where the temperature of the system could be controlled via light illumination, enabling faster transitions and higher selectivity for a new generation of the micromembranes that are found ubiquitously in MEMS and NEMS devices.

References
Falko Schmidt, Agnese Callegari, Abdallah Daddi-Moussa-Ider, Battulga Munkhbat, Ruggero Verre, Timur Shegai, Mikael Käll, Hartmut Löwen, Andrea Gambassi and Giovanni Volpe, Tunable critical Casimir forces counteract Casimir-Lifshitz attraction,
Nature Physics 19, 271-278 (2023)

Presentation by M. Selin at SBE congress, 30 June 2023

Illustration of a DNA hairpin being unzipped by an optical tweezers. (Illustration by M. Selin.)
Automating optical tweezers experiments using deep learning and custom electronics
Martin Selin
30 June 2023, 13:00 CEST

Optical tweezers are powerful tools for manipulating and studying the mechanical properties of single biomolecules, such as DNA. However, conducting such experiments manually is both time-consuming and labor-intensive limiting the amount of data collectable. In this work, we present a method to automate optical tweezers with the use of deep learning applying it to DNA pulling experiments.

A typical DNA pulling experiment can be divided into three main steps, each of which we have automated. The first is positioning of a bead in a micropipette(or secondary optical trap), second is connecting DNA of a another optically trapped bead with the bead in the micropipette and lastly the stretching of the DNA by moving the trapped bead while monitoring the force.

We have used deep learning, in particular a unet, to track beads and identify important features in the sample such as the micropipette. Combining this with realtime feedback allows the system to both trap beads and carefully position trap beads.

We demonstrate the viability of our method by stretching lambda DNA, showing human like reliability in performing the experiments. We expect our method to find use in the study of small biomolecules enabling more and faster data collection as well as longer running experiments.