Michele Zanini

Particle contact angles at fluid interfaces: pushing the boundary beyond hard uniform spherical colloids.

Micro and nanoparticles at fluid interfaces have been attracting increasing interest in the last few decades as building blocks for materials, as mechanical and structural probes for complex interfaces and as models for two-dimensional systems. The three-phase contact angle enters practically all aspects of the particle behavior at the interface: its thermodynamics (binding energy to the interface), dynamics (motion and drag at the interface) and interactions with the interface (adsorption and wetting). Moreover, many interactions among particles at the interface also strongly depend on the contact angle. These concepts have been extensively discussed for non-deformable, homogeneous and mostly spherical particles, but recent progress in particle synthesis and fabrication has instead moved in the direction of producing more complex micro and nanoscale objects, which can be responsive, deformable, heterogenous and/or anisotropic in shape, surface chemistry and material properties. These new particles have a much greater potential for applications and new science, and the study of their behavior at interfaces has only very recently started. In this paper, we critically review the current state of the art of the experimental methods available to measure the contact angle of micro and nanoparticles at fluid interfaces, indicating their strengths and limitations. We then comment on new particle systems that are currently attracting increasing interest in relation to their adsorption and assembly at fluid interfaces and discuss if and which ones of the current techniques are suited to investigate their properties at interfaces. Based on this discussion, we will finally try to indicate a direction in which new experimental methods should develop in the future to tackle the new challenges posed by the novel types of particles that more and more often are used at interfaces.

Composites reinforced via mechanical interlocking of surface-roughened microplatelets within ductile and brittle matrices

Load-bearing reinforcing elements in a continuous matrix allow for improved mechanical properties and can reduce the weight of structural composites. As the mechanical performance of composite systems are heavily affected by the interfacial properties, tailoring the interactions between matrices and reinforcing elements is a crucial problem. Recently, several studies using bio-inspired model systems suggested that interfacial mechanical interlocking is an efficient mechanism for energy dissipation in platelet-reinforced composites. While cheap and effective solutions are available at the macroscale, the modification of surface topography in micron-sized reinforcing elements still represents a challenging task. Here, we report a simple method to create nanoasperities with tailored sizes and densities on the surface of alumina platelets and investigate their micromechanical effect on the energy dissipation mechanisms of nacre-like materials. Composites reinforced with roughened platelets exhibit improved mechanical properties for both organic ductile epoxy and inorganic brittle cement matrices. Mechanical interlocking increases the modulus of toughness (area under the stress-strain curve) by 110% and 56% in epoxy and cement matrices, respectively, as compared to those reinforced with flat platelets. This interlocking mechanism can potentially lead to a significant reduction in the weight of mechanical components while retaining the structural performance required in the application field.

Robust Microcompartments with Hydrophobically Gated Shells

We report on robust synthetic microcompartments with hydrophobically gated shells that can reversibly swell and contract multiple times upon external stimuli. The gating mechanism relies on a hydrophilic-hydrophobic transition of a polymer layer that is grafted on inorganic colloidosomes using atom-transfer radical polymerization. As a result of such a transition, the initially tight hydrophobic shell becomes permeable to the diffusion of hydrophilic solutes across the microcompartment walls. Surprisingly, the microcompartments are strong enough to retain their spherical shape during several swelling and contraction cycles. This provides a powerful alternative platform for the creation of synthetic microreactors and protocells that interact with the surrounding media through a simple gating mechanism and are sufficiently robust for further engineering of increasingly complex compartmentalized structures.

Multifunctional microparticles with uniform magnetic coatings and tunable surface chemistry

Microplatelets and fibers that can be manipulated using external magnetic fields find potential applications as miniaturized probes, micromirrors in optical switches, remotely actuated micromixers and tunable reinforcements in composite materials. Controlling the surface chemistry of such microparticles is often crucial to enable full exploitation of their mechanical, optical and sensorial functions. Here, we report a simple and versatile procedure to directly magnetize and chemically modify the surface of inorganic microplatelets and polymer fibers of inherently non-magnetic compositions. As opposed to other magnetization approaches, the proposed non-aqueous sol–gel route enables the formation of a dense and homogeneous coating of superparamagnetic iron oxide nanoparticles (SPIONs) on the surface of the microparticles. Such coating provides a suitable platform for the direct chemical functionalization of the microparticles using catechol-based ligands displaying high affinity towards iron oxide surfaces. By adsorbing for example nitrodopamine palmitate (ND-PA) on the surface of hydrophilic magnetite-coated alumina platelets (Fe3O4@Al2O3) we can render them sufficiently surface active to generate magnetically responsive Pickering emulsions. We also show that microplatelets and fibers coated with a uniform iron oxide layer can be easily manipulated using low magnetic fields despite their intrinsic non-magnetic nature. These examples illustrate the potential of the proposed approach in generating functional, magnetically responsive microprobes and building blocks for several emerging applications.

Roughness-dependent tribology effects on discontinuous shear thickening

Significance Shear thickening is a ubiquitous rheological phenomenon whereby dense suspensions of particles in a fluid exhibit a viscosity increase at high shear, which can turn into a viscosity divergence [discontinuous shear thickening (DST)]. Although macroscopically well characterized, the microscopic origin of DST is still debated, especially in connection to particle surface properties, e.g., roughness and friction. We elucidate here the mechanisms underpinning DST by carrying out nanotribological measurements of the interparticle contacts of model rough colloids. We demonstrate that rough particles exhibit DST over a broader range of shear rates and for volume fractions much lower than for smooth colloids, due to interlocking of surface asperities, showing that taking an engineering-tribology approach is a powerful way to tune DST. Surface roughness affects many properties of colloids, from depletion and capillary interactions to their dispersibility and use as emulsion stabilizers. It also impacts particle–particle frictional contacts, which have recently emerged as being responsible for the discontinuous shear thickening (DST) of dense suspensions. Tribological properties of these contacts have been rarely experimentally accessed, especially for nonspherical particles. Here, we systematically tackle the effect of nanoscale surface roughness by producing a library of all-silica, raspberry-like colloids and linking their rheology to their tribology. Rougher surfaces lead to a significant anticipation of DST onset, in terms of both shear rate and solid loading. Strikingly, they also eliminate continuous thickening. DST is here due to the interlocking of asperities, which we have identified as “stick–slip” frictional contacts by measuring the sliding of the same particles via lateral force microscopy (LFM). Direct measurements of particle–particle friction therefore highlight the value of an engineering-tribology approach to tuning the thickening of suspensions.

Detachment of Rough Colloids from Liquid–Liquid Interfaces

Particle surface roughness and chemistry play a pivotal role in the design of new particle-based materials. Although the adsorption of rough particles has been studied in the literature, desorption of such particles remains poorly understood. In this work, we specifically focus on the detachment of rough and chemically modified raspberry-like microparticles from water/oil interfaces using colloidal-probe atomic force microscopy. We observe different contact-line dynamics occurring upon particle detachment (pinning vs sliding), depending on both the particle roughness and surface modification. In general, surface roughness leads to a reduction of the desorption force of hydrophobic particles into the oil and provides a multitude of pinning points that can be accessed by applying different loads. Our results hence suggest future strategies for stabilization and destabilization of Pickering emulsions and foams.

Immobilization of Colloidal Monolayers at Fluid–Fluid Interfaces

Monolayers of colloidal particles trapped at an interface between two immiscible fluids play a pivotal role in many applications and act as essential models in fundamental studies. One of the main advantages of these systems is that non-close packed monolayers with tunable inter-particle spacing can be formed, as required, for instance, in surface patterning and sensing applications. At the same time, the immobilization of particles locked into desired structures to be transferred to solid substrates remains challenging. Here, we describe three different strategies to immobilize monolayers of polystyrene microparticles at water–decane interfaces. The first route is based on the leaking of polystyrene oligomers from the particles themselves, which leads to the formation of a rigid interfacial film. The other two rely on in situ interfacial polymerization routes that embed the particles into a polymer membrane. By tracking the motion of the colloids at the interface, we can follow in real-time the formation of the polymer membranes and we interestingly find that the onset of the polymerization reaction is accompanied by an increase in particle mobility determined by Marangoni flows at the interface. These results pave the way for future developments in the realization of thin tailored composite polymer-particle membranes.