Active Matter Alters the Growth Dynamics of Coffee Rings
Tuğba Andaç, Pascal Weigmann, Sabareesh K. P. Velu, Erçağ Pinçe, Agnese Callegari, Giorgio Volpe, Giovanni Volpe & Agnese Callegari
Soft Matter 15(7), 1488—1496 (2019)
How particles are deposited at the edge of evaporating droplets, i.e. the coffee ring effect, plays a crucial role in phenomena as diverse as thin-film deposition, self-assembly, and biofilm formation. Recently, microorganisms have been shown to passively exploit and alter these deposition dynamics to increase their survival chances under harshening conditions. Here, we show that, as the droplet evaporation rate slows down, bacterial mobility starts playing a major role in determining the growth dynamics of the edge of drying droplets. Such motility-induced dynamics can influence several biophysical phenomena, from the formation of biofilms to the spreading of pathogens in humid environments and on surfaces subject to periodic drying. Analogous dynamics in other active matter systems can be exploited for technological applications in printing, coating, and self-assembly, where the standard coffee-ring effect is often a nuisance.
Controlling the dynamics of colloidal particles by critical Casimir forces
Alessandro Magazzù, Agnese Callegari, Juan Pablo Staforelli, Andrea Gambassi, Siegfried Dietrich & Giovanni Volpe
Soft Matter (2019)
Critical Casimir forces can play an important role for applications in nano-science and nano-technology, owing to their piconewton strength, nanometric action range, fine tunability as a function of temperature, and exquisite dependence on the surface properties of the involved objects. Here, we investigate the effects of critical Casimir forces on the free dynamics of a pair of colloidal particles dispersed in the bulk of a near-critical binary liquid solvent, using blinking optical tweezers. In particular, we measure the time evolution of the distance between the two colloids to determine their relative diffusion and drift velocity. Furthermore, we show how critical Casimir forces change the dynamic properties of this two-colloid system by studying the temperature dependence of the distribution of the so-called first-passage time, i.e., of the time necessary for the particles to reach for the first time a certain separation, starting from an initially assigned one. These data are in good agreement with theoretical results obtained from Monte Carlo simulations and Langevin dynamics.
Active Matter Alters the Growth Dynamics of Coffee Rings Agnese Callegari, Tugba Andaç, Pascal Weigmann, Sabareesh K. Velu, Erçag Pince, Giorgio Volpe & Giovanni Volpe
LAOP – Latin America Optics & Photonics Congress, Lima, Peru
12-15 November 2018
Abstract: We show that bacterial mobility starts playing a major role in determining the growth dynamics of the edge of drying droplets, as the droplet evaporation rate slows down.
Optical Trapping and Optical Manipulation
Giovanni Volpe & Agnese Callegari
Tutorial at LAOP – Latin America Optics & Photonics Congress, Lima, Peru
12-15 November 2018
Description: This course will review the theoretical underpinnings of optical trapping and optical manipulation; a review of recent applications; and provide a hands-on tutorial on the use of computational methods to simulate optical trapping and the motion of optically trapped particles.
Time: 09:00 – 13:00 Location: INICTEL-UNI, Lima, Peru
Clustering of Janus Particles in Optical Potential Driven by Hydrodynamic Fluxes
S. Masoumeh Mousavi, Sabareesh K. P. Velu, Agnese Callegari, Luca Biancofiore & Giovanni Volpe
Self-organisation is driven by the interactions between the individual components of a system mediated by the environment, and is one of the most important strategies used by many biological systems to develop complex and functional structures. Furthermore, biologically-inspired self-organisation offers opportunities to develop the next generation of materials and devices for electronics, photonics and nanotechnology. In this work, we demonstrate experimentally that a system of Janus particles (silica microspheres half-coated with gold) aggregates into clusters in the presence of a Gaussian optical potential and disaggregates when the optical potential is switched off. We show that the underlying mechanism is the existence of a hydrodynamic flow induced by a temperature gradient generated by the light absorption at the metallic patches on the Janus particles. We also perform simulations, which agree well with the experiments and whose results permit us to clarify the underlying mechanism. The possibility of hydrodynamic-flux-induced reversible clustering may have applications in the fields of drug delivery, cargo transport, bioremediation and biopatterning.
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Microscopic engine powered by critical demixing
Falko Schmidt, Alessandro Magazzù, Agnese Callegari, Luca Biancofiore, Frank Cichos & Giovanni Volpe
Physical Review Letters 120(6), 068004 (2018)
We experimentally demonstrate a microscopic engine powered by the local reversible demixing of a critical mixture. We show that, when an absorbing microsphere is optically trapped by a focused laser beam in a sub-critical mixture, it is set into rotation around the optical axis of the beam because of the emergence of diffusiophoretic propulsion. This behavior can be controlled by adjusting the optical power, the temperature, and the criticality of the mixture.