arXiv subjects

Lucio Isa

Publications and source records attributed to Lucio Isa.

At least 37 records · Page 2Linked to original sources

arXiv2023

Minimal numerical ingredients describe chemical microswimmers's 3D motion

The underlying mechanisms and physics of catalytic Janus microswimmers is highly complex, requiring details of the associated phoretic fields and the physiochemical properties of catalyst, particle, boundaries, and the fuel used. Therefore, developing a minimal (and more general) model capable of capturing the overall dynamics of these autonomous particles is highly desirable. In the presented work, we demonstrate that a coarse-grained dissipative particle-hydrodynamics model is capable of describing the behaviour of various chemical microswimmer systems. Specifically, we show how a competing balance between hydrodynamic interactions experienced by a squirmer in the presence of a substrate, gravity, and mass and shape asymmetries can reproduce a range of dynamics seen in different experimental systems. We hope that our general model will inspire further synthetic work where various modes of swimmer motion can be encoded via shape and mass during fabrication, helping to realise the still outstanding goal of microswimmers capable of complex 3-D behaviour

cond-mat.soft↗

arXiv2023

Tuning electrostatic interactions of colloidal particles at oil-water interfaces with organic salts

Monolayers of colloidal particles at oil-water interfaces readily crystalize owing to electrostatic repulsion, which is often mediated through the oil. However, little attempts exist to control it using oil-soluble electrolytes. We probe the interactions amongst charged hydrophobic micospheres confined at a water/hexadecane interface and show that repulsion can be continuously tuned over orders of magnitude upon introducing minor amounts of an organic salt into the oil. Our results show that charged groups at the particle/oil interface are subject to an associative discharging mechanism, analogous to the charge regulation kinetics observed for charged colloids in non-polar solvents.

cond-mat.soft↗

arXiv2022

The effect of curvature on the diffusion of colloidal bananas

Anisotropic colloidal particles exhibit complex dynamics which play a crucial role in their functionality, transport and phase behaviour. In this work, we investigate the two-dimensional diffusion of smoothly curved colloidal rods -- also known as colloidal bananas -- as a function of their opening angle, α. We measure the translational and rotational diffusion coefficients of the particles with opening angles ranging from 0° (straight rods) to nearly 360°(closed rings). In particular, we find that the anisotropic diffusion of the particles varies non-monotonically with their opening angle and that the axis of fastest diffusion switches from the long to the short axis of the particles when α>180°. We also find that the rotational diffusion coefficient of nearly closed rings is approximately an order of magnitude higher than that of straight rods of the same length. Finally, we show that the experimental results are consistent with Slender Body Theory, indicating that the dynamical behavior of the particles arises primarily from their local drag anisotropy. These results highlight the impact of curvature on the Brownian Motion of elongated colloidal particles, which must be taken into account when seeking to understand the behaviour of curved colloidal particles.

cond-mat.soft↗

arXiv2022

Self-propelling colloidal finite state machines

Endowing materials with physical intelligence holds the key for a progress leap in robotic systems. In spite of the growing success for macroscopic devices, transferring these concepts to the microscale presents several challenges connected to the lack of suitable fabrication and design techniques, and of internal response schemes that connect the materials' properties to the function of an autonomous unit. Here, we realize self-propelling colloidal clusters which behave as simple finite state machines, i.e. systems built to possess a finite number of internal states connected by reversible transitions and associated to distinct functions. We produce these units via capillary assembly combining hard polystyrene colloids with two different types of thermo-responsive microgels. The clusters, actuated by spatially uniform AC electric fields, adapt their shape and dielectric properties, and consequently their propulsion, via reversible temperature-induced transitions controlled by light. The different transition temperatures for the two microgels enable three distinct dynamical states corresponding to three illumination intensity levels. The sequential reconfiguration of the microgels affects the velocity and shape of the active trajectories according to a pathway defined by tailoring the clusters' geometry during assembly. The demonstration of these simple systems indicates an exciting route to build more complex units with broader reconfiguration schemes and multiple responses towards the realization of autonomous systems with physical intelligence at the colloidal scale.

cond-mat.soft↗

arXiv2022

Printing on particles: combining two-photon nanolithography and capillary assembly to fabricate multi-material microstructures

Additive manufacturing at the micro- and nanoscale has seen a recent upsurge to suit the increasing demand for more elaborate structures. However, the integration and precise placement of multiple distinct materials at small scales remain a challenge. To this end, we combine here the directed capillary assembly of colloidal particles and two-photon direct laser writing (DLW) to realize a new class of multi-material microstructures. We use DLW both to fabricate 3D micro-templates to guide the capillary assembly of soft- and hard colloids, and to link well-defined arrangements of polystyrene or silica particles produced with capillary assembly, a process we term "printing on particles". The printing process is based on automated particle recognition algorithms and enables the user to connect colloids into one- and two-dimensional tailored structures, including particle clusters and lattices of varying symmetry and composition, using commercial photo-resists (IP-L or IP-PDMS). Once printed and developed, the structures can be easily harvested and re-dispersed in water. The flexibility of our method allows the combination of a wide range of materials into complex structures, which we envisage will boost the realization of new systems for a broad range of fields, including microrobotics, micromanipulation and metamaterials.

cond-mat.soft↗

arXiv2022

Log it: How to fit an active Brownian particle's mean squared displacement with improved parameter estimation

The active Brownian particle (ABP) model is widely used to describe the dynamics of active matter systems, such as Janus microswimmers. In particular, the analytical expression for an ABP's mean-squared-displacement (MSD) is useful as it provides a means to describe the essential physics of a self-propelled, spherical Brownian particle. However, the truncated or 'short-time' form of the MSD equation is typically fitted, which can lead to significant problems in parameter estimation. Furthermore, heteroscedasticity and the often statistically dependent observations of an ABP's MSD lead to a situation where standard ordinary least squares (OLS) regression will obtain biased estimates and unreliable confidence intervals. Here, we propose to revert to always fitting the full expression of an ABP's MSD at short timescales, using bootstrapping to construct confidence intervals of the fitted parameters. Additionally, after comparison between different fitting strategies, we propose to extract the physical parameters of an ABP using its mean logarithmic squared displacement (MLSD). These steps improve the estimation of an ABP's physical properties, and provide more reliable confidence intervals, which are critical in the context of a growing interest in the interactions of microswimmers with confining boundaries and the influence on their motion.

cond-mat.soft↗

arXiv2022

Tracking Janus microswimmers in 3D with Machine Learning

Advancements in artificial active matter heavily rely on our ability to characterise their motion. Yet, the most widely used tool to analyse the latter is standard wide-field microscopy, which is largely limited to the study of two-dimensional motion. In contrast, real-world applications often require the navigation of complex three-dimensional environments. Here, we present a Machine Learning (ML) approach to track Janus microswimmers in three dimensions, using Z-stacks as labelled training data. We demonstrate several examples of ML algorithms using freely available and well-documented software, and find that an ensemble decision tree-based model (Extremely Randomised Decision Trees) performs the best at tracking the particles over a volume spanning a depth of more than 40 $μ$m. With this model, we are able to localise Janus particles with a significant optical asymmetry from standard wide-field microscopy images, bypassing the need for specialised equipment and expertise such as that required for digital holographic microscopy. We expect that ML algorithms will become increasingly prevalent by necessity in the study of active matter systems, and encourage experimentalists to take advantage of this powerful tool to address the various challenges within the field.

cond-mat.soft↗

arXiv2022

Steering self-organisation through confinement

Self-organisation is the spontaneous emergence of spatio-temporal structures and patterns from the interaction of smaller individual units. Examples are found across many scales in very different systems and scientific disciplines, from physics, materials science and robotics to biology, geophysics and astronomy. Recent research has highlighted how self-organisation can be both mediated and controlled by confinement. Confinement occurs through interactions with boundaries, and can function as either a catalyst or inhibitor of self-organisation. It can then become a means to actively steer the emergence or suppression of collective phenomena in space and time. Here, to provide a common framework for future research, we examine the role of confinement in self-organisation and identify overarching scientific challenges across disciplines that need to be addressed to harness its full scientific and technological potential. This framework will not only accelerate the generation of a common deeper understanding of self-organisation but also trigger the development of innovative strategies to steer it through confinement, with impact, e.g., on the design of smarter materials, tissue engineering for biomedicine and crowd management.

nlin.AO↗

arXiv2022

In-situ imaging of the three-dimensional shape of soft responsive particles at fluid interfaces by atomic force microscopy

The reconfiguration of soft, deformable particles upon adsorption at the interface between two fluids underpins many aspects of their dynamics and interactions, ultimately controlling the macroscopic properties of particle monolayers of relevance for materials, such as particle-stabilized emulsions and foams, and processes, e. g. particle-based lithography. In spite of its importance, experimentally determining the three-dimensional shape of soft particles at fluid interfaces with high resolution remains an elusive task. In this work, we take poly(N-isopropylacrylamide) (pNIPAM) microgels as model soft particles and demonstrate that their conformation at the interface between an aqueous and an oil phase can be fully reconstructed by means of in-situ atomic force microscopy (AFM) imaging. We show that imaging the particle topography from both sides of the interface allows one to characterize the in-plane deformation of the particle under the action of interfacial tension and to visualize the occurrence of asymmetric swelling in the two fluids. Additionally, the technique enables investigating different fluid phases and particle architectures, as well as studying in situ the effect of temperature variations on particle conformation. We envisage that these results open up an exciting range of possibilities to provide microscopic insights between the single-particle behavior of soft objects at fluid interfaces and macroscopic material properties of relevance for applications and fundamental studies alike.

cond-mat.soft↗

arXiv2021

Influence of the interfacial tension on the microstructural and mechanical properties of microgels at fluid interfaces

Microgels are soft colloidal particles constituted by cross-linked polymer networks with a high potential for applications. In particular, after adsorption at a fluid interface, interfacial tension provides two-dimensional (2D) confinement for microgel monolayers and drives the reconfiguration of the particles, enabling their deployment in foam and emulsion stabilization and in surface patterning for lithography, sensing and optical materials. However, most studies focus on systems of fluids with a high interfacial tension, e.g. alkanes/ or air/water interfaces, which imparts similar properties to the assembled monolayers. Here, instead, we compare two organic fluid phases, hexane and methyl tert-butyl ether, which have markedly different interfacial tension ($γ$) values with water and thus tune the elasticity and deformation of adsorbed microgels. We rationalize how $γ$ controls the single-particle morphology, which consequently modulates the structural and mechanical response of the monolayers at varying interfacial compression. Specifically, when $γ$ is low, the microgels are less deformed within the interface plane and their polymer networks can rearrange more easily upon lateral compression, leading to softer monolayers. Selecting interfaces with different surface energy offers an additional control to customize the 2D assembly of soft particles, from the fine-tuning of particle size and interparticle spacing to the tailoring of mechanical properties.

cond-mat.soft↗

arXiv2021

Microrobots from Toposelective Nanoparticle Attachment

Microrobots hold promise for applications ranging from targeted delivery to enhanced mixing at the microscale. However, current fabrication techniques suffer from limited throughput and material selection. Here, we demonstrate a versatile route enabling the synthesis of microrobots from off-the-shelf micro- and nano-particles. Our protocol hinges on the toposelective attachment of photocatalytic nanoparticles onto microparticles, exploiting a multi-functional polymer and a Pickering-wax emulsification step, to yield large quantities of photo-responsive active Janus particles. The polymer presents both silane and nitrocatechol groups, binding silica microspheres to a range of metal oxide nanoparticles. The Pickering-wax emulsions protect part of the microspheres' surface, enabling asymmetric functionalization, as required for self-propulsion. The resulting photocatalytic microrobots display a characteristic orientation-dependent 3D active motion upon UV illumination, different to that conventionally described in the literature. By connecting the extensive library of heterogeneous nanoparticle photocatalysts with the nascent field of active matter, this versatile material platform lays the groundwork towards designer microrobots, which can swim by catalysing a broad range of chemical reactions with potential for future applications.

physics.app-ph↗

arXiv2021

The effect of internal architecture on the assembly of soft particles at fluid interfaces

Monolayers of soft colloidal particles confined at fluid interfaces have been attracting increasing interest for fundamental studies and applications alike. However, establishing the relation between their internal architecture, which is controlled during synthesis, and their structural and mechanical properties upon interfacial confinement, which define the monolayer's properties, remains an elusive task. Here, we propose a comprehensive study elucidating this relation for a system of microgels with tunable architecture. We synthesize core-shell microgels, whose soft core can be chemically degraded in a controlled fashion, yielding particles ranging from analogues of standard batch-synthesized to completely hollow microgels after total core removal. We characterize the internal structure of these particles, their swelling properties in bulk and their morphologies upon adsorption at an oil-water interface via a combination of numerical simulations and complementary experiments. In particular, we confirm that hollow microgels are mechanically stable in bulk aqueous conditions and that the progressive removal of the core leads to a significant flattening of the microgels, which become disk-like particles, at the interface. At low compression, the mechanical response of the monolayer is dominated by the presence of loosely crosslinked polymers forming a corona surrounding the particle within the interfacial plane, regardless of the presence of a core. By contrast, at high compression, the absence of a core enables the particles to deform in the direction orthogonal to the interface. These findings shed new light on which structural features of soft particles determine their interfacial behaviour, enabling new design strategies for tailored materials.

cond-mat.soft↗

arXiv2020

Near-zero surface pressure assembly of rectangular lattices of microgels at fluid interfaces for colloidal lithography

Understanding and engineering the self-assembly of soft colloidal particles (microgels) at liquid-liquid interfaces is broadening their use in colloidal lithography. Here, we present a new route to assemble rectangular lattices of microgels at near zero surface pressure relying on the balance between attractive quadrupolar capillary interactions and steric repulsion among the particles at water/oil interfaces. These self-assembled rectangular lattices are obtained for a broad range of particles and, after deposition, can be used as lithography masks to obtain regular arrays of vertically aligned nanowires via wet and dry etching processes.

cond-mat.soft↗

arXiv2020

Reconfigurable Artificial Microswimmers with Internal Feedback

Micron-size self-propelling particles are often proposed as synthetic models for biological microswimmers, yet they lack internally regulated adaptation, which is central to the autonomy of their biological counterparts. Conversely, adaptation and autonomy can be encoded in larger-scale soft-robotic devices, but transferring these capabilities to the colloidal scale remains elusive. Here, we create a new class of responsive microswimmers, powered by induced-charge electrophoresis, which can adapt their motility to external stimuli via an internal feedback. Using sequential capillary assembly, we fabricate deterministic colloidal clusters comprising soft thermoresponsive microparticles, which, upon spontaneous reconfiguration, induce motility changes, such as adaptation of the clusters' propulsion velocity and reversal of its direction. We rationalize the response in terms of a coupling between self-propulsion and variations of particle shape and dielectric properties. Harnessing those allows for strategies to achieve local dynamical control with simple illumination patterns, revealing exciting opportunities for the development of new tactic active materials.

cond-mat.soft↗

arXiv2020

Feedback-Controlled Active Brownian Colloids with Space-Dependent Rotational Dynamics

The non-thermal nature of self-propelling colloids offers new insights into non-equilibrium physics. The central mathematical model to describe their trajectories is active Brownian motion, where a particle moves with a constant speed, while randomly changing direction due to rotational diffusion. While several feedback strategies exist to achieve position-dependent velocity, the possibility of spatial and temporal control over rotational diffusion, which is inherently dictated by thermal fluctuations, remains untapped. Here, we decouple rotational diffusion from thermal noise. Using external magnetic fields and discrete-time feedback loops, we tune the rotational diffusivity of active colloids above and below its thermal value at will and explore a rich range of phenomena including anomalous diffusion, directed transport, and localization. These findings add a new dimension to the control of active matter, with implications for a broad range of disciplines, from optimal transport to smart materials.

cond-mat.soft↗

arXiv2020

Broadband Mie-driven random quasi-phase-matching

High-quality crystals without inversion symmetry are the conventional platform to achieve optical frequency conversion via three wave-mixing. In bulk crystals, efficient wave-mixing relies on phase-matching configurations, while at the micro- and nano-scale it requires resonant mechanisms that enhance the nonlinear light-matter interaction. These strategies commonly result in wavelength-specific performances and narrowband applications. Disordered photonic materials, made up of a random assembly of optical nonlinear crystals, enable a broadband tunability in the random quasi-phase-matching (RQPM) regime and do not require high-quality materials. Here, we combine resonances and disorder by implementing RQPM in Mie-resonant spheres of a few microns realized by the bottom-up assembly of barium titanate nano-crystals. The measured second harmonic generation (SHG) reveals a combination of broadband and resonant wave mixing, in which Mie resonances drive and enhance the SHG, while the disorder keeps the phase-matching conditions relaxed. This new phase-matching regime can be described by a random walk in the SHG complex plane whose step lengths depend on the local field enhancement within the micro-sphere. Our nano-crystals assemblies provide new opportunities for tailored phase-matching at the micro-scale, beyond the coherence length of the bulk crystal. They can be adapted to achieve frequency conversion from the near-ultraviolet to the infrared ranges, they are low-cost and scalable to large surface areas.

physics.optics↗

arXiv2020

Microscale Marangoni Surfers

We apply laser light to induce the asymmetric heating of Janus colloids adsorbed at water-oil interfaces and realize active micrometric "Marangoni surfers". The coupling of temperature and surfactant concentration gradients generates Marangoni stresses leading to self-propulsion. Particle velocities span four orders of magnitude, from microns/s to cm/s, depending on laser power and surfactant concentration. Experiments are rationalized by finite elements simulations, defining different propulsion regimes relative to the magnitude of the thermal and solutal Marangoni stress components.

cond-mat.soft↗

arXiv2020

Disentangling the roles of roughness, friction and adhesion in discontinuous shear thickening by means of thermo-responsive particles

Dense suspensions of colloidal or granular particles can display pronounced non-Newtonian behaviour, such as discontinuous shear thickening (DST) and shear jamming (SJ). The essential contribution of particle surface roughness and adhesive forces confirms that stress-activated contacts can play a key role in these phenomena. By employing a system of microparticles coated by responsive polymers that allow friction, adhesion, and surface roughness to be selectively and independently tuned as a function of temperature, we offer a way to disentangle these contributions. We find that DST occurs at lower shear rates when friction and adhesion between particles are enhanced at high temperatures. Additionally, the temperature-responsive polymers provide lubricity at low temperatures that can mask surface roughness. The link between single-particle properties and macroscopic rheology is elucidated via lateral force microscopy, which reveals the nature of rheologically relevant contact conditions. In situ temperature tuning during shear allows contact conditions to be modified, and DST to be switched on and off on demand. These findings strengthen our understanding of the microscopic parameters affecting DST and identify new routes for the design of smart, non-Newtonian fluids.

cond-mat.soft↗