Autotitrator based on an Arduino Open Source Pump.

Acid-base titration is a quantitative analysis that enables knowing the quantity of acidic or basic groups present in a solution sample. It consists in the addition of base or acid to the solution sample while monitoring the pH to reach a neutral pH. The titration can be automated and here we present a low cost Arduino based Open Source Pump (OSPump) modified to act as an automated titrator with an obsolete but reliable Metrohm 713 pH meter. Our device is 50 times less expensive than second hand units from the pH meter manufacturer and inherently open to customization. We present two validation cases of study, including the lipolysis of a vegetable olive oil in water emulsion, characterized by the OSPump Titrator.

Automation of an atomic force microscope via Arduino.

The Atomic Force Microscopy is a very versatile technique that allows to characterize surfaces by acquiring topographies with sub-nanometer resolution. This technique often overcomes the problems and capabilities of electron microscopy when characterizing few nanometers thin coatings over solid substrates. They are expensive, in the half million dollar range for standard units, and therefore it is often difficult to upgrade to new units with improved characteristics. One of these improvements, motorization and automation of the measurements is very interesting to sample different parts of a substrate in an unattended way. Here we report a low cost upgrade under 60 $ to a Dimension 3000 AFM based on a control unit using an Arduino Leonardo. It enables to acquire dozens or hundreds of images automatically by mimicking keyboard shortcuts and interfacing the AFM PCI card.

Hot Brownian Motion of Thermoresponsive Microgels in Optical Tweezers Shows Discontinuous Volume Phase Transition and Bistability.

Microgels are soft microparticles that often exhibit thermoresponsiveness and feature a transformation at a critical temperature, referred to as the volume phase transition temperature. Whether this transformation occurs as a smooth or as a discontinuous one is still a matter of debate. This question can be addressed by studying individual microgels trapped in optical tweezers. For this aim, composite particles are obtained by decorating  Poly-N-isopropylacrylamide (pNIPAM) microgels with iron oxide nanocubes. These composites become self-heating when illuminated by the infrared trapping laser, performing hot Brownian motion within the trap. Above a certain laser power, a single decorated microgel features a volume phase transition that is discontinuous, while the usual continuous sigmoidal-like dependence is recovered after averaging over different microgels. The collective sigmoidal behavior enables the application of a power-to-temperature calibration and provides the effective drag coefficient of the self-heating microgels, thus establishing these composite particles as potential micro-thermometers and micro-heaters. Moreover, the self-heating microgels also exhibit an unexpected and intriguing bistability behavior above the critical temperature, probably due to partial collapses of the microgel. These results set the stage for further studies and the development of applications based on the hot Brownian motion of soft particles.

Sequential capillarity-assisted particle assembly in a microfluidic channel.

Colloidal patterning enables the placement of a wide range of materials into prescribed spatial arrangements, as required in a variety of applications, including micro- and nano-electronics, sensing, and plasmonics. Directed colloidal assembly methods, which exploit external forces to place particles with high yield and great accuracy, are particularly powerful. However, currently available techniques require specialized equipment, which limits their applicability. Here, we present a microfluidic platform to produce versatile colloidal patterns within a microchannel, based on sequential capillarity-assisted particle assembly (sCAPA). This new microfluidic technology exploits the capillary forces resulting from the controlled motion of an evaporating droplet inside a microfluidic channel to deposit individual particles in an array of traps microfabricated onto a substrate. Sequential depositions allow the generation of a desired spatial layout of colloidal particles of single or multiple types, dictated solely by the geometry of the traps and the filling sequence. We show that the platform can be used to create a variety of patterns and that the microfluidic channel easily allows surface functionalization of trapped particles. By enabling colloidal patterning to be carried out in a controlled environment, exploiting equipment routinely used in microfluidics, we demonstrate an easy-to-build platform that can be implemented in microfluidics labs.

Microgels at interfaces, from mickering emulsions to flat interfaces and back.

In this review, we cover the topic of p(NIPAM) based microgels at interfaces, revisiting classical studies in light of the newest ones. In particular, we focus on their use as emulsifiers in the so-called mickering emulsions, i.e. Pickering emulsion stabilized by soft particles. Given the complexity of the experimental characterization and simulation of these soft particles at interfaces, the review is structured in progressive complexity levels, until we reach the highly interesting and promising responsiveness to stimuli of mickering emulsions. We start from the lowest level of complexity, the current understanding of the behavior of single microgels confined at a flat interface. Then, we discuss their collective behavior upon crowding, their responsiveness at interfaces, and their macroscopic properties as microgel films. Once we have the necessary characterization tools, we proceed to discuss the complex and convoluted picture of responsive mickering emulsions. The way is rough, with current controversial and contradicting studies, but it holds promising results as well. We state open questions worth of being tackled by the Soft Matter community, and we conclude that it is worth the trouble of continuing after the master theory of microgel interfacial activity, as it will pave the way to widely adopt responsive mickering emulsions as the worthy Pickering emulsion successors.

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.

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 fluctuations. 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.

Self-templating assembly of soft microparticles into complex tessellations.

Encoding Archimedean and non-regular tessellations in self-assembled colloidal crystals promises unprecedented structure-dependent properties for applications ranging from low-friction coatings to optoelectronic metamaterials1-7. Yet, despite numerous computational studies predicting exotic structures even from simple interparticle interactions8-12, the realization of complex non-hexagonal crystals remains experimentally challenging13-18. Here we show that two hexagonally packed monolayers of identical spherical soft microparticles adsorbed at a liquid-liquid interface can assemble into a vast array of two-dimensional micropatterns, provided that they are immobilized onto a solid substrate one after the other. The first monolayer retains its lowest-energy hexagonal structure and acts as a template onto which the particles of the second monolayer are forced to rearrange. The frustration between the two lattices elicits symmetries that would not otherwise emerge if all the particles were assembled in a single step. Simply by varying the packing fraction of the two monolayers, we obtain not only low-coordinated structures such as rectangular and honeycomb lattices, but also rhomboidal, hexagonal and herringbone superlattices encoding non-regular tessellations. This is achieved without directional bonding, and the structures formed are equilibrium structures: molecular dynamics simulations show that these structures are thermodynamically stable and develop from short-range repulsive interactions, making them easy to predict, and thus suggesting avenues towards the rational design of complex micropatterns.

Active Brownian Motion with Orientation-Dependent Motility: Theory and Experiments.

Combining experiments on active colloids, whose propulsion velocity can be controlled via a feedback loop, and the theory of active Brownian motion, we explore the dynamics of an overdamped active particle with a motility that depends explicitly on the particle orientation. In this case, the active particle moves faster when oriented along one direction and slower when oriented along another, leading to anisotropic translational dynamics which is coupled to the particle's rotational diffusion. We propose a basic model of active Brownian motion for orientation-dependent motility. On the basis of this model, we obtain analytical results for the mean trajectories, averaged over the Brownian noise for various initial configurations, and for the mean-square displacements including their non-Gaussian behavior. The theoretical results are found to be in good agreement with the experimental data. Orientation-dependent motility is found to induce significant anisotropy in the particle displacement, mean-square displacement, and non-Gaussian parameter even in the long-time limit. Our findings establish a methodology for engineering complex anisotropic motilities of active Brownian particles, with a potential impact in the study of the swimming behavior of microorganisms subjected to anisotropic driving fields.

Poly-N-isopropylacrylamide Nanogels and Microgels at Fluid Interfaces.

The confinement of colloidal particles at liquid interfaces offers many opportunities for materials design. Adsorption is driven by a reduction of the total free energy as the contact area between the two liquids is partially replaced by the particle. From an application point of view, particle-stabilized interfaces form emulsions and foams with superior stability. Liquid interfaces also effectively confine colloidal particles in two dimensions and therefore provide ideal model systems to fundamentally study particle interactions, dynamics, and self-assembly. With progress in the synthesis of nanomaterials, more and more complex and functional particles are available for such studies. In this Account, we focus on poly(N-isopropylacrylamide) nanogels and microgels. These are cross-linked polymeric particles that swell and soften by uptaking large amounts of water. The incorporated water can be partially expelled, causing a volume phase transition into a collapsed state when the temperature is increased above approximately 32 °C. Soft microgels adsorbed to liquid interfaces significantly deform under the influence of interfacial tension and assume cross sections exceeding their bulk dimensions. In particular, a pronounced corona forms around the microgel core, consisting of dangling chains at the microgel periphery. These polymer chains expand at the interface and strongly affect the interparticle interactions. The particle deformability therefore leads to a significantly more complex interfacial phase behavior that provides a rich playground to explore structure formation processes. We first discuss the characteristic "fried-egg" or core-corona morphology of individual microgels adsorbed to a liquid interface and comment on the dependence of this interfacial morphology on their physicochemical properties. We introduce different theoretical models to describe their interfacial morphology. In a second part, we introduce how ensembles of microgels interact and self-assemble at liquid interfaces. The core-corona morphology and the possibility to force these elements into overlap upon compression results in a complex phase behavior with a phase transition between microgels with extended and collapsed coronae. We discuss the influence of the internal particle architecture, also including core-shell microgels with rigid cores, on the phase behavior. Finally, we present new routes for the realization of more complex structures, resulting from multiple deposition protocols and from engineering the interaction potential of the individual particles.

Effect of the 3D Swelling of Microgels on Their 2D Phase Behavior at the Liquid-Liquid Interface.

We investigate soft, temperature-sensitive microgels at fluid interfaces. Though having an isotropic, spherical shape in bulk solution, the microgels become anisotropic upon adsorption. The structure of microgels at interfaces is described by a core-corona morphology. Here, we investigate how changing temperature across the microgel volume phase transition temperature, which leads to swelling/deswelling of the microgels in the aqueous phase, affects the phase behavior within the monolayer. We combine compression isotherms, atomic force microscopy imaging, multiwavelength ellipsometry, and computer simulations. At low compression, the interaction between adsorbed microgels is dominated by their highly stretched corona and the phase behavior of the microgel monolayers is the same. The polymer segments within the interface lose their temperature-sensitivity because of the strong adsorption to the interface. At high compression, however, the portions of the microgels that are located in the aqueous side of the interface become relevant and prevail in the microgel interactions. These portions are able to collapse and, consequently, the isostructural phase transition is altered. Thus, the temperature-dependent swelling perpendicular to the interface ("3D") affects the compressibility parallel to the interface ("2D"). Our results highlight the distinctly different behavior of soft, stimuli-sensitive microgels as compared to rigid nanoparticles.

Creating gradients of amyloid fibrils from the liquid-liquid interface.

We report a method to deposit amyloid fibrils on a substrate creating gradients in orientation and coverage on demand. For this purpose, we adapt a colloidal self-assembly method at liquid-liquid interfaces to deposit amyloid fibrils on a substrate from the water-hexane interface, while simultaneously compressing it. The amyloid fibril layers orient perpendicularly to the compression, ranging from isotropic to nematic distributions. We furthermore observe reproducible transitions from a monolayer to a bilayer and from a bilayer to multilayers with increasing surface pressures. The creation of each new layer is accompanied by a systematic drop in the structural order of the system, which is however regained upon further compression. This method shows great potential for overcoming the thin-film engineering challenges associated with the manipulation of sticky amyloid fibrils, and allows their ex situ visualisation under compression at the fluid-fluid interface, a situation relevant to understand the propagation of amyloid-related diseases, their functional role in biological systems, and their potential for technological applications.

Mechanical phase inversion of Pickering emulsions via metastable wetting of rough colloids.

The possibility to invert emulsions from oil-in-water to water-in-oil (or vice versa) in a closed system, i.e. without any formulation change, remains an open fundamental challenge with many opportunities for industrial applications. Here, we propose a mechanism that exploits particle surface roughness to induce metastable wetting and obtain mechanically-responsive Pickering emulsions. We postulate that the phase inversion is driven by an in situ switch of the particle wettability from metastable positions at the interface following the input of controlled mechanical energy. Oil-in-water emulsions can be prepared at low energy using mildly hydrophobic rough colloids, which are dispersed in water and weakly pinned at the interface, and switched to water-in-oil emulsions by a second emulsification at higher energy, which triggers the relaxation of the particle contact angle. The same principle is demonstrated for the complementary emulsions using mildly hydrophilic colloids initially dispersed in oil. Our experiments and simulations support that the delicate interplay between particle surface design during synthesis and the energy of the emulsification process can encode a kinetic pathway for the phase inversion. Both organic and inorganic nanoparticles can be used, allowing for the future implementation of our strategy in a broad range of smart industrial formulations.

Microgels Adsorbed at Liquid-Liquid Interfaces: A Joint Numerical and Experimental Study.

Soft particles display highly versatile properties with respect to hard colloids and even more so at fluid-fluid interfaces. In particular, microgels, consisting of a cross-linked polymer network, are able to deform and flatten upon adsorption at the interface due to the balance between surface tension and internal elasticity. Despite the existence of experimental results, a detailed theoretical understanding of this phenomenon is still lacking due to the absence of appropriate microscopic models. In this work, we propose an advanced modeling of microgels at a flat water/oil interface. The model builds on a realistic description of the internal polymeric architecture and single-particle properties of the microgel and is able to reproduce its experimentally observed shape at the interface. Complementing molecular dynamics simulations with in situ cryo-electron microscopy experiments and atomic force microscopy imaging after Langmuir-Blodgett deposition, we compare the morphology of the microgels for different values of the cross-linking ratios. Our model allows for a systematic microscopic investigation of soft particles at fluid interfaces, which is essential to develop predictive power for the use of microgels in a broad range of applications, including the stabilization of smart emulsions and the versatile patterning of surfaces.

Tunable 2D binary colloidal alloys for soft nanotemplating.

The realization of non-close-packed nanoscale patterns with multiple feature sizes and length scales via colloidal self-assembly is a highly challenging task. We demonstrate here the creation of a variety of tunable particle arrays by harnessing the sequential self-assembly and deposition of two differently sized microgel particles at the fluid-fluid interface. The two-step process is essential to achieve a library of 2D binary colloidal alloys, which are kinetically inaccessible by direct co-assembly. These versatile binary patterns can be exploited for a range of end-uses. Here we show that they can for instance be transferred to silicon substrates, where they act as masks for the metal-assisted chemical etching of binary arrays of vertically aligned silicon nanowires (VA-SiNWs) with fine geometrical control. In particular, continuous binary gradients in both NW spacing and height can be achieved. Notably, these binary VA-SiNW platforms exhibit interesting anti-reflective properties in the visible range, in agreement with simulations. The proposed strategy can also be used for the precise placement of metallic nanoparticles in non-close-packed arrays. Sequential depositions of soft particles enable therefore the exploration of complex binary patterns, e.g. for the future development of substrates for biointerfaces, catalysis and controlled wetting.

Particles adsorbed at various non-aqueous liquid-liquid interfaces.

Particles adsorbed at liquid interfaces are commonly used to stabilise water-oil Pickering emulsions and water-air foams. The fundamental understanding of the physics of particles adsorbed at water-air and water-oil interfaces is improving significantly due to novel techniques that enable the measurement of the contact angle of individual particles at a given interface. The case of non-aqueous interfaces and emulsions is less studied in the literature. Non-aqueous liquid-liquid interfaces in which water is replaced by other polar solvents have properties similar to those of water-oil interfaces. Nanocomposites of non-aqueous immiscible polymer blends containing inorganic particles at the interface are of great interest industrially and consequently more work has been devoted to them. By contrast, the behaviour of particles adsorbed at oil-oil interfaces in which both oils are immiscible and of low dielectric constant (ε<3) is scarcely studied. Hydrophobic particles are required to stabilise these oil-oil emulsions due to their irreversible adsorption, high interfacial activity and elastic shell behaviour.

Compression and deposition of microgel monolayers from fluid interfaces: particle size effects on interface microstructure and nanolithography.

Controlling the microstructure of monolayers of microgels confined at a water/oil interface is the key to their successful application as nanolithography masks after deposition on a solid substrate. Previous work demonstrated that compression of the monolayer can be used to tune the microgel arrangement and to explore the full two-dimensional area-pressure phase diagram of the particles trapped at the interface. Here, we explore a new size range, using microgels with 210 nm and 1.45 µm bulk diameters, respectively. We start by investigating the properties of isolated particles in situ at the interface by freeze-fracture cryo-SEM, and after deposition using an atomic force microscope. We then study their collective behavior in a compressed monolayer and highlight significant differences in terms of the accessible structural phases and their transitions. More specifically, the larger microgels behave similar to colloids with a hard core and a soft polymeric shell, exhibiting capillarity driven clustering at a large specific area and a solid-solid phase transition between two hexagonal lattices at higher compressions. The smaller particles instead show no aggregation and a smooth transition from a hexagonal lattice to a dense disordered monolayer. Finally, we demonstrate that the larger microgels can be effectively turned into masks for the fabrication of vertically aligned silicon nanowires by means of metal-assisted chemical etching. These findings highlight the subtle interplay between particle architecture, adsorption and interactions at the interface, the understanding and harnessing of which are at the basis of their successful use as nanopatterning tools.

Isostructural solid-solid phase transition in monolayers of soft core-shell particles at fluid interfaces: structure and mechanics.

We have studied the complete two-dimensional phase diagram of a core-shell microgel-laden fluid interface by synchronizing its compression with the deposition of the interfacial monolayer. Applying a new protocol, different positions on the substrate correspond to different values of the monolayer surface pressure and specific area. Analyzing the microstructure of the deposited monolayers, we discovered an isostructural solid-solid phase transition between two crystalline phases with the same hexagonal symmetry, but with two different lattice constants. The two phases corresponded to shell-shell and core-core inter-particle contacts, respectively; with increasing surface pressure the former mechanically failed enabling the particle cores to come into contact. In the phase-transition region, clusters of particles in core-core contacts nucleate, melting the surrounding shell-shell crystal, until the whole monolayer moves into the second phase. We furthermore measured the interfacial rheology of the monolayers as a function of the surface pressure using an interfacial microdisk rheometer. The interfaces always showed a strong elastic response, with a dip in the shear elastic modulus in correspondence with the melting of the shell-shell phase, followed by a steep increase upon the formation of a percolating network of the core-core contacts. These results demonstrate that the core-shell nature of the particles leads to a rich mechanical and structural behavior that can be externally tuned by compressing the interface, indicating new routes for applications, e.g. in surface patterning or emulsion stabilization.

A simple strategy to improve the interfacial activity of true Janus gold nanoparticles: a shorter hydrophilic capping ligand.

Janus gold nanoparticles (JPs) of ∼4 nm-diameter half functionalized with 1-hexanethiol as a hydrophobic capping ligand exhibit significantly higher interfacial activity, reproducibility and rheological response when the other half is functionalized with 1,2-mercaptopropanediol (JPs-MPD) than with 2-(2-mercaptoethoxy)ethanol (JPs-MEE), both acting as hydrophilic capping ligands. The interfacial pressure measured by pendant drop tensiometry reaches 50 mN m(-1) and 35 mN m(-1) for the JPs-MPD at the water/air and water/decane interface, respectively. At the same area per particle, the JPs-MEE reveal significantly lower interfacial pressure: 15 mN m(-1) and 5 mN m(-1) at the water/air and water/decane interface, respectively. Interfacial dilatational rheology measurements also show an elastic shell behaviour at higher compression states for JPs-MPD while the JPs-MEE present near-zero elasticity. The enhanced interfacial activity of JPs-MPD is explained in terms of chemical and hydration differences between the MPD and MEE ligands, where MPD has a shorter hydrocarbon chain and twice as many hydroxyl terminal groups as MEE.

Fully Tunable Silicon Nanowire Arrays Fabricated by Soft Nanoparticle Templating.

We demonstrate a fabrication breakthrough to produce large-area arrays of vertically aligned silicon nanowires (VA-SiNWs) with full tunability of the geometry of the single nanowires and of the whole array, paving the way toward advanced programmable designs of nanowire platforms. At the core of our fabrication route, termed "Soft Nanoparticle Templating", is the conversion of gradually compressed self-assembled monolayers of soft nanoparticles (microgels) at a water-oil interface into customized lithographical masks to create VA-SiNW arrays by means of metal-assisted chemical etching (MACE). This combination of bottom-up and top-down techniques affords excellent control of nanowire etching site locations, enabling independent control of nanowire spacing, diameter and height in a single fabrication route. We demonstrate the fabrication of centimeter-scale two-dimensional gradient photonic crystals exhibiting continuously varying structural colors across the entire visible spectrum on a single silicon substrate, and the formation of tunable optical cavities supported by the VA-SiNWs, as unambiguously demonstrated through numerical simulations. Finally, Soft Nanoparticle Templating is combined with optical lithography to create hierarchical and programmable VA-SiNW patterns.