Teresa Brugarolas

Janus particles at an oil–water interface

We study the behaviour of Janus particles at an oil–water interface. Amphiphilic Janus particles exhibit attractive interactions at the fluid–fluid interface. The attractive interactions are likely due to the pinning of contact line around the diffuse boundary between the two hemispheres. The undulation of the three-phase contact line around Janus particles leads to quadrupolar capillary interactions, which we confirm by measuring the interparticle forces. We also show that Janus particles with two negatively charged-hydrophilic hemispheres show repulsive interactions.

Avoiding Cracks in Nanoparticle Films

A new method utilizing subsequent depositions of thin crack-free nanoparticle layers is demonstrated to avoid the formation of cracks within silica nanoparticle films. Using this method, films can be assembled with thicknesses exceeding the critical cracking values. Explanation of this observed phenomenon is hypothesized to mainly arise from chemical bond formation between neighboring silica nanoparticles. Application of this method for fabricating crack-free functional structures is demonstrated by producing crack-free Bragg reflectors that exhibit structural color.

Mechanical reinforcement of nanoparticle thin films using atomic layer deposition.

Thin films composed of nanoparticles exhibit synergistic properties, making them useful for numerous advanced applications. Nanoparticle thin films (NTFs), however, have a very low resistance to mechanical loading and abrasion, presenting a major bottleneck to their widespread use and commercialization. High-temperature sintering has been shown to improve the mechanical durability of NTFs on inorganic substrates; however, these high-temperature processes are not amenable to organic substrates. In this study, we demonstrate that the mechanical durability of TiO(2)/SiO(2) nanoparticle layer-by-layer (LbL) films on glass and polycarbonate substrates can be drastically improved using atomic layer deposition (ALD) at a relatively low temperature. The structure and physical properties of ALD-treated TiO(2)/SiO(2) nanoparticle LbL films are studied using spectroscopic ellipsometry, UV-vis spectroscopy, contact angle measurements, and nanoindentation. The composition of TiO(2)/SiO(2) LbL films as a function of ALD-cycle number is determined through solution ellipsometry, enabling the determination of the characteristic pore size of nanoparticle thin films. Mechanical durability is also investigated by abrasion tests, showing that the robustness of ALD-treated nanoparticle films is comparable to that of thermally calcined films. More importantly, ALD-treated nanoparticle films retain the original functionality of the TiO(2)/SiO(2) LbL films, such as superhydrophilicity and antireflection properties, demonstrating the utility of ALD as a reinforcement method for nanoparticle thin films.

Directed assembly of particles using microfluidic droplets and bubbles

Assembly of particles into three dimensional structures is critical for a variety of advanced applications including photonics, optics, catalysis, MEMS, drug delivery and biosensing. Needless to say, a precise control over the structure and properties of three-dimensional particle assemblies is essential in maximizing the functionality that is afforded by the particles in these structures. One method that enables rapid, inexpensive and potentially scalable assembly of particles involves using fluid droplets and bubbles as structure directing agents. Recent advances in microfluidics allow for the formation of highly uniform and structured droplets and bubbles that can be used to direct the assembly of particles into three dimensional structures. In this review, we introduce the recent developments in using microfluidic techniques to generate highly uniform and complex droplets and bubbles, which are subsequently used to direct the assembly of various nano and microparticles. We also highlight a number of functional supraparticles that have been shown to exhibit unique photonic and sensing properties. We conclude this review by providing an outlook on the current challenges and opportunities that need to be addressed for these microfluidic-based approaches to have a broader impact as widely applicable methods for directed assembly of particles.

Using shape anisotropy to toughen disordered nanoparticle assemblies.

Assemblies of disordered nanoparticles constitute an important class of materials that have numerous applications in energy conversion and storage, electronics, photonics, and sensing. One major roadblock that limits the widespread utilization of disordered nanoparticle assemblies (DNAs) is their poor damage tolerance; they fracture under small loads and, thus, have low toughness. The absence of fundamental understanding on the mechanical behavior and failure mechanism of disordered nanoparticle assemblies makes it even more challenging to develop new strategies to toughen these structures without compromising their mechanical strength. Here we show the formation of shear bands, highly localized regions of mechanical strain that prelude fracture, in disordered assemblies of spherical nanoparticles, which bear striking resemblance to the deformation mechanism of a different class of disordered materials, metallic glasses. We demonstrate that anisotropic nanoparticles greatly suppress shear band formation and toughen nanoparticle packings without sacrificing their strength, implying that tuning constituent anisotropy can be used to enhance toughness in disordered packings of nanoparticles.

Beauty of Lotus is More than Skin Deep: Highly Buoyant Superhydrophobic Films

We develop highly buoyant superhydrophobic films that mimic the three-dimensional structure of lotus leaves. The high buoyancy of these structure stems from mechanically robust bubbles that significantly reduce the density of the superhydrophobic films. These highly buoyant superhydrophobic films stay afloat on water surface while carrying a load that is more than 200 times their own weight. In addition to imparting high buoyancy, the incorporation of robust hydrophilic bubbles enables the formation of free-standing structures that mimic the water-collection properties of Namib Desert beetle. We believe the incorporation of robust bubbles is a general method that opens up numerous possibilities in imparting high buoyancy to different structures that needs to stay afloat on water surfaces and can potentially be used for the fabrication of lightweight materials. (Image on the upper left reproduced with permission from Yong, J.; Yang, Q.; Chen, F.; Zhang, D.; Du, G.; Si, J.; Yun, F.; Hou, X. A Bioinspired Planar Superhydrophobic Microboat. J. Micromech. Microeng. 2014, 24, 035006).

Tailoring and Understanding the Mechanical Properties of Nanoparticle-Shelled Bubbles

One common approach to generate lightweight materials with high specific strength and stiffness is the incorporation of stiff hollow microparticles (also known as bubbles or microballoons) into a polymeric matrix. The mechanical properties of these composites, also known as syntactic foams, greatly depend on those of the hollow microparticles. It is critical to precisely control the properties of these bubbles to fabricate lightweight materials that are suitable for specific applications. In this paper, we present a method to tailor the mechanical properties and response of highly monodisperse nanoparticle-shelled bubbles using thermal treatment. We characterize the mechanical properties of individual as-assembled bubbles as well as those of thermally treated ones using nanoindentation and quantitative in situ compression tests. As-assembled bubbles display inelastic response, whereas thermally treated bubbles behave elastically. We also show that the stiffness and strength of bubbles are enhanced significantly, as much as 12 and 14 times that of the as-assembled bubbles, respectively, via thermal treatment. We complement the experimental results with finite element analysis (FEA) to understand the effect of shell thickness nonuniformity as well as the inelasticity on the mechanical response and fracture behavior of these bubbles. We demonstrate that the failure mechanism of bubbles incorporated into a polymer composite depends on the structure of the bubbles.