Lucio Isa

Measuring single-nanoparticle wetting properties by freeze-fracture shadow-casting cryo-scanning electron microscopy

Nanoparticles at fluid interfaces are central to a rapidly increasing range of cutting-edge applications, including drug delivery, uptake through biological membranes, emulsion stabilization and the fabrication of nanocomposites. Understanding nanoscale wetting is a challenging issue, still unresolved for individual nanoparticles, and is essential in designing nanoparticle-building blocks with controlled surface properties. The core information about the structural and thermodynamic properties of particles at fluid interfaces is enclosed in the three-phase contact angle θ. Here we present a novel in situ method, on the basis of freeze-fracture shadow-casting cryo-scanning electron microscopy, that allows the measurement of contact angles of individual nanoparticles with 10 nm diameter, and thus greatly surpasses the current state of the art. We study hydrophilic and hydrophobic, organic and inorganic nanoparticles, demonstrating general applicability to systems of fundamental and applied nanotechnological interest. Significant heterogeneity in the wetting of nanoparticles is observed.

Shear Banding and Flow-Concentration Coupling in Colloidal Glasses

We report experiments on hard-sphere colloidal glasses that show a type of shear banding hitherto unobserved in soft glasses. We present a scenario that relates this to an instability due to shear-concentration coupling, a mechanism previously thought unimportant in these materials. Below a characteristic shear rate γ(c) we observe increasingly nonlinear and localized velocity profiles. We attribute this to very slight concentration gradients in the unstable flow regime. A simple model accounts for both the observed increase of γ(c) with concentration, and the fluctuations in the flow.

Particle lithography from colloidal self-assembly at liquid-liquid interfaces.

Particle lithography has been extensively used as a robust and cost-effective method to produce large-area, close-packed arrays of nanometer scale features. Many technological applications, including biosensing, require instead non-close-packed patterns in order to avoid cross-talk between the features. We present a simple, scalable, single-step particle lithography process that employs colloidal self-assembly at liquid-liquid interfaces (SALI) to fabricate regular, open particle lithography masks, where the size of the features (40 to 500 nm) and their separation can be independently controlled between 3 and 10 particle diameters. Finally we show how the process can be practically employed to produce diverse biosensing structures.

Unraveling the 3D Localization and Deformation of Responsive Microgels at Oil/Water Interfaces: A Step Forward in Understanding Soft Emulsion Stabilizers

Responsive microgels are deformable submicrometer cross-linked polymeric hydrogel particles that are used as a novel class of emulsion stabilizers. Their flexibility and the triggering of conformational changes by external stimuli lead to several advantages compared to rigid particles used in conventional Pickering emulsions. Despite their rapidly increasing use, several key aspects relating to microgel microstructure and localization at liquid interfaces are still unexplored. We present here a novel characterization that employs freeze-fracture shadow-casting cryo-SEM to disclose quantitative 3D information on the deformation and protrusion of microgels at water/oil interfaces. Despite the bulk pH response (swelling), we report here the unexpected absence of size and vertical position changes as a function of pH at liquid interfaces and interpret the results using simple arguments that link the particle interfacial activity, solvation, and internal deformation. These results pave the way to a deeper understanding of a novel class of soft materials.

Microscopic Mechanism for Shear Thickening of Non-Brownian Suspensions

We propose a simple model, supported by contact-dynamics simulations as well as rheology and friction measurements, that links the transition from continuous to discontinuous shear thickening in dense granular pastes to distinct lubrication regimes in the particle contacts. We identify a local Sommerfeld number that determines the transition from Newtonian to shear-thickening flows, and then show that the suspensions volume fraction and the boundary lubrication friction coefficient control the nature of the shear-thickening transition, both in simulations and experiments.

Non-equilibrium nature of two-dimensional isotropic and nematic coexistence in amyloid fibrils at liquid interfaces

Two-dimensional alignment of shape-anisotropic colloids is ubiquitous in nature, ranging from interfacial virus assembly to amyloid plaque formation. The principles governing two-dimensional self-assembly have therefore long been studied, both theoretically and experimentally, leading, however, to diverging fundamental interpretations on the nature of the two-dimensional isotropic-nematic phase transition. Here we employ single-molecule atomic force microscopy, cryogenic scanning electron microscopy and passive probe particle tracking to study the adsorption and liquid crystalline ordering of semiflexible β-lactoglobulin fibrils at liquid interfaces. Fibrillar rigidity changes on increasing interfacial density, with a maximum caused by alignment and a subsequent decrease stemming from crowding and domain bending. Coexistence of nematic and isotropic regions is resolved and quantified by a length scale-dependent order parameter S(2D)(d). The nematic surface fraction increases with interfacial fibril density, but depends, for a fixed interfacial density, on the initial bulk concentration, ascribing the observed two-dimensional isotropic-nematic coexistence to non-equilibrium phenomena.

Thermal-lensing measurement of particle thermophoresis in aqueous dispersions

We show that thermophoresis (particle drift driven by thermal gradients) in aqueous solutions can be measured by using an all-optical thermal-lensing setup, where a temperature gradient is set by a near-infrared laser beam with no need of light-absorbing dyes. After discussing the principles of the method, we study by numerical simulation the nature and extent of parasitic thermal-convection effects, and we describe an optical setup designed to limit them. We finally present preliminary results on thermophoresis in micellar solutions and colloidal dispersions.

Direct Measurement of Contact Angles of Silica Particles in Relation to Double Inversion of Pickering Emulsions

In an alkane-water system containing submicrometer silica particles at high pH, double emulsion inversion from oil-in-water (o/w) to water-in-oil (w/o) to oil-in-water can be effected by increasing the concentration of a dichain cationic surfactant in water. The contact angle θ of the particles at the planar oil-water interface has been measured directly using freeze-fracture shadow-casting cryo-scanning electron microscopy, enabling single-particle measurements of high accuracy. θ passes through a maximum with respect to surfactant concentration. It is shown that particles undergo a hydrophilic-hydrophobic-hydrophilic transition corresponding closely to the o/w-w/o-o/w transformation observed in emulsions. These results unequivocally link the single-particle contact angles to the type of particle-stabilized emulsion, confirming macroscopic emulsion inversion on the microscopic level.

Wall slip and flow of concentrated hard-sphere colloidal suspensions

We present a comprehensive study of the slip and flow of concentrated colloidal suspensions using cone-plate rheometry and simultaneous confocal imaging. In the colloidal glass regime, for smooth, nonstick walls, the solid nature of the suspension causes a transition in the rheology from Herschel–Bulkley (HB) bulk flow behavior at large stress to a Bingham-like slip behavior at low stress, which is suppressed for sufficient colloid-wall attraction or colloid-scale wall roughness. Visualization shows how the slip-shear transition depends on gap size and the boundary conditions at both walls and that partial slip persist well above the yield stress. A phenomenological model, incorporating the Bingham slip law and HB bulk flow, fully accounts for the behavior. Microscopically, the Bingham law is related to a thin (subcolloidal) lubrication layer at the wall, giving rise to a characteristic dependence of slip parameters on particle size and concentration. We relate this to the suspension’s osmotic pressure an...

The Compressibility of pH- Sensitive Microgels at the Oil- Water Interface: Higher Charge Leads to Less Repulsion

pH-responsive microgels are unique stabilizers for stimuli-sensitive emulsions that can be broken on demand by changing the pH value. However, recent experiments have indicated that electrostatic interactions play a different role to that in conventional Pickering emulsions. The influence of charges on the interactions between microgels at the oil-water interface is now described. Compression isotherms of microgels with different charge density and architecture were determined in a Langmuir trough, and counter-intuitive results were obtained: Charged microgels can be compressed more easily than uncharged microgels. The compressibility of microgels is thus not determined by direct Coulomb repulsion. Instead, the different swelling of the microgels in the charged and the uncharged states is proposed to be the key parameter.