Session S26: Hierarchical Bioinspired Materials

Calculation of effective properties of composite materials with periodic microstructures using the Asymptotic Homogenization Method

Mean-field estimates of effective material properties of periodic structures using the Mori-Tanaka (MT) method fail to capture the interparticle interactions at higher inclusion volume fractions. The finite element based asymptotic homogenization method (AHM), serves as a robust numerical tool in this regard. Assuming there exists a substantial separation of length scales between the macroscopic and microscopic structures, the perturbations of the potential field caused due to the presence of inclusion under a macroscopic loading are used to predict the effective property. This method is utilised to study the effective electrical conductivity in fibrous systems and compared against existing MT estimates. For spherical inclusions, the study revealed that MT estimate and AHM agree well at volume fractions less than 0.4. However, near maximum packing fractions, AHM results fared significantly better than MT when compared with known asymptotic forms (Keller 1963). The current work aims at implementing the proposed AHM method to structures with aligned spheroidal inclusions of various aspect ratios and conductivity ratios, thus providing a more generalized approach to predict the effective electrical conductivity.   

Microporous nanoparticle emulsion thermosets for multi-material, multifunctional porous nanocomposites

We have developed microporous nanoparticle emulsion thermosets (MiNET), a new class of nanocomposites made from epoxies and nanoparticles, a liquid porogen, and a small quantity of surfactant. These ingredients form an intermediate between a conventional surfactant a Pickering emulsion to create a bicontinuous network of oil and epoxy composite throughout the processing. After a room temperature cure and usage of different functional nanoparticles, based on the performance requirements of a given application, it is possible to design a composite with a range of functionality like flexibility, inertness, and conductivity. Further extraction of the oil phase through rinsing, MiNETs can be converted into porous (30~60% open volume) structures without considerable volume shrinkage (~1~5%). The pore size (between 100~10,000 nm) and chemical functionality of the pores is tunable by the constituent nanoparticles. Simultaneously, the matrix resin can change mechanical properties and use of silicone nanoparticles as a filler can establish flexible behavior. These novel thermosets molded into various complex forms. For example, we have demonstrated parallel-processed electrospray emitters at micrometer scale and Carbon fiber nanocomposites with MiNET as a matrix.   

Spontaneous "wave ripples" through sequential wrinkling interference

The coupling between fluid motion, both unidirectional and periodic, and sediment transport, gives rise to the familiar yet fascinating, self-organisation processes responsible for the formation of `ripples' in sand beds and beach dunes. Analogous patterns are found in folded, layered rock formations, developing a range of orthogonal to non-orthogonal interference patterns. Striking periodic and aperiodic structures also emerge in biological morphogenesis, for instance in the epicuticular topography of certain insect wings, such as dragonflies or cicadae, modulating appearance and function, such as structural colour and superhydrophobicity. Here, we report the formation of `sand ripple' patterns by the sequential superposition of non-orthogonal surface waves excited by the spontaneous buckling of polymeric bilayers. A PDMS slab is uni-axially stretched and then subjected to plasma oxidation, forming a thin glassy skin. Upon strain release, a first wrinkling generation is formed, with prescribed wavelength and amplitude. Interference is achieved by superposing the second wrinkling generation at a prescribed angle, termed `compression angle'. This sequential wrinkling approach provides a facile and scalable framework to induce tunable undulated and checkerboard patterns, by varying layer skin and strain parameters. Albeit of a different nature and micronscale compared to the familiar sedimentary ripples caused by gentle wave oscillations, we find commonalities in their topography, defects and bifurcations. The patterns are rationalised in terms of a defect density that depends on the relative angle between generations, and a constant in-plane bending angle that depends on skin thickness. A minimal wave summation model enables the design of ripple and checkerboard surfaces by tuning material properties and fabrication process, guiding surface fabrication, mimicking naturally-occurring patterns, with potential practical applications.   

Deformations of compliant textured surfaces due to surface stress

Protrusions or textures on a compliant solid surface will tend to deform, flatten, and develop rounded corners when the surface is in contact with a liquid or air. Considering initially rectangular ridges of height h and width w, this deformation has been studied experimentally and theoretically for nearly planar surfaces, where the aspect ratio is small, h/w << 1. We develop a microfabrication technique to produce compliant textures with large amplitude and extend these investigations to compliant textures with higher aspect ratio, h/w ~ 1. We experimentally measure texture flattening and rounding of sharp corners for two-dimensional ridges and three-dimensional pillars, and observe large deformations. We compare the results to previous theoretical predictions for low-aspect-ratio structures and comment on how the intial wavelength λ, height h, and width w change the amount of the final deformation. Finally, we discuss the implications for surface stress.   

Flexible Fibrillar Surface Array Morphology as a Function of Inter-Fibril Interactions

Mesoscale polymer ribbons (MSPs) are unique high aspect ratio filaments formed by controlled evaporative self-assembly. Upon release into aqueous solution, MSPs exhibit 3D morphologies sensitive to their geometry, material properties, and environmental interactions. This structural tunability offers a novel means to construct bioinspired hierarchical assemblies. Previous work has provided insight into the physics governing single MSP configurations, but there remains a limited understanding of the effects of inter-filament and substrate interactions on fibrillar shape and behavior. To study these influences, we fabricate surface-bound MSP filament arrays and harness interfacial forces to control their morphology. We use fluorescent microscopy to monitor the evolution of these structures as a function of ambient pH, salinity, and dissolved polymer. Using computational analysis, we correlate changes in the radius of curvature and inter-ribbon separation to environmental conditions. This work elucidates the impact of interfacial interactions and assembly phenomena in multi-ribbon mesoscale structures, underpinning the MSP as an emerging platform in soft, bioinspired hierarchical assemblies.   

Self-Assembly of 2D Host-Guest Pore Networks via Entropy Compartmentalization

On the atomic scale, open host–guest (HG) structures (where one guest atom or molecule is enveloped by a cage of host atoms) are common. Zeolites are one example where a porous host structure traps and orients guest molecules. To instead use HG mechanisms on the colloidal scale to assemble useful structures for applications like photonics and particle adsorption, a deeper understanding of why HG structures assemble is needed. In this work, we show that athermal binary mixtures of simple concave star particles and convex polygon guest particles form HG networks. We show the reason for this assembly is due to entropy compartmentalization. In these systems the host particles are orientationally locked and trade a decrease in entropy to increase the entropy of the guest particles which rotate freely, thereby maximizing the system entropy. We also show that these pore networks are robust to small perturbations of the host shape, and that adsorption of hard polygons onto the porous 2D host network follows classical adsorption theory. Our results elucidate the thermodynamics of host-guest systems that are robust to shape perturbation and can be used in hard particle adsorption onto the 2D surface.   

Size-dependent polarizability of mesoscopic ionic clusters during assembly

Mesoscopic clusters composed of oppositely charged particles are ubiquitous in synthetic and biological soft materials. Using coarse-grained simulations, we demonstrate that the ionic polarizability of electrically neutral ionic clusters, and hence their induced-dipole interactions, vary as charged particles aggregate into the clusters during assembly. By applying an external electrical field, we measure the ionic polarizability as a function of the cluster size, which is characterized by the number of charged particles inside the cluster. Four different models are examined, namely, clusters of binary charges, clusters of neutral polyampholytes, polyelectrolytes in a collapsed conformation, and chloroform-containing polyelectrolyte complexes. The mobility of the constituent charged particles in response to the electrical field is the root cause for the size-dependent polarizability of these mesoscopic clusters, which are fundamentally different from macroscopic objects. Importantly, the findings again emphasize that the effective interaction of the ionic clusters are hierarchical and time-dependent in nature.   

Viscosity Metamaterials

Metamaterials are composite materials that are typically engineered by assembling their constituents at a scale smaller than the characteristic length associated with their response. They have traditionally been used to design exotic properties including structural, optical, thermal, and acoustic properties that are otherwise not achievable from the conventional materials. Here, we introduce a new class of metamaterials- viscosity metamaterials- created by applying acoustic perturbations to a shear thickening suspension. Specifically, we show that the perturbations can drive large viscosity oscillations spanning orders of magnitude at a timescale that is much smaller than the timescale of global material flow. This behavior enables us to construct metamaterials whose viscosity is a combination of thickened high viscosity and fully dethickened low viscosity states. We show in a phase diagram that the boundaries of the viscosity metamaterial is determined by the interplay between the applied strain rate, acoustic power, and the timescales of perturbations. We envision that these novel viscosity metamaterials can be used in numerous applications ranging from soft robotics to microfluidics, and from 3D printing to engineering exotic fluids.   

Perylene-based Self-Assembly of Chloroplast-like Lamellae for Artificial Photosynthesis

Water oxidation is a vital mechanism performed by photosynthetic organisms, converting solar energy into oxygen. So far, the majority of man-made systems undergoing water oxidation after irradiation is limited into dimers, experiencing strong charge recombination. Here we report a self-assembled structure composed of multi-perylene bisimides (PBIs) and a polyoxometalate (POM) catalyst as a guest, building highly ordered three-dimensional (3D) non-covalent assembled lamellas in water. We perform Transient Absorption (TA) spectroscopy to capture the ultrafast (100 fs) formation of symmetry-breaking charge separated (SB-CS) states between PBI units, mimicking the photosynthetic special pair. Afterwards, we observe an efficient 8 ps hole transfer to the POM catalyst, creating PBI(-) and POM(+) sites. The bi-exponential recombination leads to the presence of long-lived charges in the catalytic sites, optimal for efficient water splitting. Furthermore, ultrafast TA anisotropy is employed to track the motion of the charges in real time. Our data show that the charges are rapidly move in the 3D space preventing fast recombination.   

Bundlemers-peptide nanoparticles for hierarchical material assembly

 Bundlemers are a series of computationally designed peptides that form monodisperse cylindrical particles through aqueous self-assembly into antiparallel homotetrameric coiled coils. The robust coiled-coil-forming motif has extremely tunable surface chemistry while still remaining a monodisperse particle.  Using this tunability, we have used bundlemers as macromonomers to create stiff and extremely high aspect ratio supramolecular polymers through covalent chemistry between the neighboring bundlemer ends.  The rigid rods (persistence length > 10 microns) exhibit a cross section of 2 nm with controllable length of up to microns. Through computational design, the net charge of the rods can be controlled in order to manipulate solution behavior. The rigid rods show a characteristic shear thinning rheological behavior and, when highly charged at low salt, produce gels due to repulsive electrostatic interactions. With salt screening, the charged rod particles form nematic liquid crystal phases.  A combination of rheology, x-ray scattering, and microscopy measurements was used for solution characterization.  Processing of the rod suspensions into fibers will also be discussed.   

Capsids under compression

Icosahedral shells play important functional roles in many systems, including protein-shelled viruses, bacterial microcompartments, and synthetic nanocages. The reversible self-assembly process that leads to formation of such shells has been extensively studied during the past two decades, both theoretically and experimentally. In comparison, we have relatively little knowledge of their disassembly, in particular when disassembly is driven by mechanical forcing. In this talk we use dynamical Monte Carlo simulations to investigate icosahedral shells undergoing compression between two parallel walls. We particularly focus on the onset and propagation of cracks leading to shell disassembly. We quantify the increase in brittleness of the shells with the subunit stiffness and we follow the inhomogeneous nucleation and interaction of the propagating cracks under steady load. Finally, we apply cyclic load to the shells and measure the degree of recovery after the load is removed.   

Microscale Mechanics and Structural Organization of Cross-linked Actin Networks

Actin, a globular protein, is a major component of the cytoskeleton. In the presence of ATP and magnesium, G-actin polymerizes into filamentous actin (F-actin), which plays crucial structural and mechanical roles in cell stability, motion, replication, and muscle contraction. Most of these mechanically driven structural changes in cells stem from the complex viscoelastic nature of cross-linked actin filaments. How actin networks respond to local nonlinear stresses is not well understood. Here, we use optical tweezers to impart local nonlinear strains and measure the resulting stresses during and following the strain of cross-linked actin networks. We actively drive a microsphere 10 micrometers through cross-linked actin networks at a constant speed and measure the resistive force that the network exerts on the bead during and following strain. We determine the viscoelastic response of the phalloidin stabilized actin network by varying the concentration of a cross-linker. We simultaneously image the network via fluorescence LSCM to characterize the networks' structural change and heterogeneity as the density of cross-linking protein increases. Our results shed light on how cells undergo morphological changes through varying crosslinker densities.   

Field driven transition of artificial membraneless organells: from spheres to discoids to prolate ellipsoids

The shape of an object can give them a distinctive transport advantage, evolutionarily helpful in complex organisms. For example, the discoidal shape of red blood cells and their ability to deform allow them to pass through channels with a diameter smaller than their own. Inspired by such non-classical shapes in natural systems, we studied external field driven shape transitions in one of the ubiquitous biological materials: membraneless organelles (MLOs). MLOs are a class of cellular compartments formed by liquid-liquid phase separation of biopolymers that do not have, otherwise commonly found, the lipid membrane. We found that the artificial MLOs made by phase separation of polyelectrolytes (also known as coacervates) show peculiar shape transitions under an external electric field. These transform from their equilibrium spherical shape to short-lived discoid before elongating into prolate ellipsoid aligned orthogonal to the applied field direction. We attribute these peculiar shape transitions to electrohydrodynamic flows combined with exceptional dielectric properties of polyelectrolyte MLOs. Such field-induced non-classical shape transitions can be directly utilized in the de-jamming of microfluidic colloidal flows.