Session Y57: Physics of Bioinspired Soft Materials II

From Disordered to Quasi-Ordered Structures: Bioinspired Non-Iridescent Structural Colors from Phase-Separated Polymer Blend Films

Structural colors, have gained lot of prominence in the recent years, owing to their vibrancy, durability and environment friendliness with special emphasis on angle-independent (non-iridescent) colors. Numerous studies on using colloids and block copolymers for fabricating angle-independent colors have been reported in literature. We demonstrate temperature-induced phase-separation in polymer blend films as a strategy to achieve angle-independent colors closely mimicking colors in nature (blue colors of barbs in Eastern bluebird and white color of beetle scales). We have employed PS/PMMA blend system as a model system of study. Morphology of the films ranging from disordered to quasi-ordered to ordered structures with varying length scales were fabricated by controlling the thermodynamics of polymer blend phase-separation in terms of composition, thermal quench depth and film thickness. Characterization of the films using SEM, USAXS and UV-Visible spectroscopy revealed that the resultant color of the films arose from disordered and quasi-ordered structures based on incoherent and coherent scattering respectively. The strategy employed is thus compatible via a roll-to-roll assembly enabling us to fabricate these colors on a large-scale.   

Self-assembly and dynamics of freely-jointed emulsion polymers

We present the self-assembly process of emulsion polymers, which differ from colloidosomes, or colloidal polymers, in that they are flexible and deformable. When two emulsion droplets labeled with complementary DNA meet, the balance of DNA hybridization energy and droplet deformation energy yields an equilibrium patch size\footnote{Feng \textit{et. al}. (2013). Soft Matter, 9(41).}. Varying the concentration of DNA allows us to tune the number of droplet-droplet bonds to assemble dimers, polymers, or even cross-linked networks. A random combinatoric model for picking binding partners, given an input valency distribution, is in quantitative agreement with the statistics of the observed droplet configurations, suggesting the system achieves the steady-state limit. Under tailored assembly conditions, we find that ~96\% of droplets assemble into either linear or branched polymers. Analyzing the dynamics of only the linear chains, we find that they are freely-jointed and that the end-to-end length fluctuations follow the 2D Flory model of polymer scaling. These polymers are then collapsed into compact clusters via secondary interactions, allowing us to monitor the distribution of folded structures for a given chain length $N$.   

Self-assembly of coarse-grained DNA origami subunits

DNA origami is an fruitful method to create a wide range of simple
and complex objects on the nanometer scale.
By folding DNA into truncated tetrahedra ("triangles") with
protrusions and recessions on their outer edges, one can add
attractive interaction between these triangles.
This way, the triangles can self-assemble into icosahedra much the way
virus coat proteins do, but on larger, more accessible length- and
timescales.
These icosahedra could in turn be an ideal candidate for producing
functional materials because of the unprecedented control over the
subunit structure and functionality.

To guide the design of these building blocks we perform molecular
dynamics simulations of coarse-grained, rigid triangles with reactive
patches.
By probing the self-assembly kinetics and assembly products as a
function of the reactive patch properties we attempt to determine the
optimal design strategy for the DNA origami building blocks in order
to build octahedra and icosahedra.
In particular, we will focus on the influence of the interaction
strength on the assembly yield.   

Morphological Investigation of Lysozyme-Liquid Crystal dried Droplet Pattern formation

Drying liquid drops are important physical systems in understanding the basic science and technology. A sessile droplet forms innumerable patterns when evaporating with a free boundary condition. A recent study of drying dynamics of a lyotropic liquid crystal has illuminated details of the evaporation dynamics. This paper will report on the pattern formation of a dried droplet of a mixture of lysozyme, a nematic thermotropic liquid crystal (LC) 5CB (4-cyano-4’-pentylbiphenyl), and deionized water as a function of LC concentrations for different boundary conditions. We use cross-polarizing microscopy to investigate the optical activity and morphology of the final dried droplet at various sample angles. The overall morphology reveals a complex pattern of protein-rich regions separated by nearly pure LC material that varies with distance from the droplet edge and LC concentration. The quantitative analysis is done by ImageJ/Fiji software yielding parameters that characterize the morphology and are related to the initial composition of the mixture. This paper demonstrates the utility of using a thermotropic LC as a probe material in a protein mixture, revealing new information on the protein aggregation.   

Theory of non-equilibrium signal processing in hydrogels

Living organisms store and differentiate information by processing complex dynamic environmental signals. Current hydrogel-based systems are constrained to an in-phase coupling between environmental changes and swelling or contraction. In order to integrate non-equilibrium complex signal processing capabilities into hydrogels, we model the interplay between transport of multiple chemical signals, their complexation with the hydrogel network, and the resulting dynamic poroelastic responses. The theory guides the design of a bio-inspired experimental hydrogel system that can detect and report features of the environment it would otherwise be unable to differentiate. In particular, it (i) distinguishes between signals and their propagation direction, converts (ii) transient stimuli to continuous responses or (iii) continuous stimuli to transient responses, and (iv) filters signals or amplifies them. Our combined theoretical and experimental approach provides new functionalities for engineering applications where complex signal processing is critical, ranging from sensors to soft robotics.   

Biomimicking Vitamin B12: A Co Phthalocyanine Pyridine Axial Ligand Coordinated Catalyst for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is of extreme importance in energy converting systems since oxygen is cheap and abundant electron (e) acceptor. The complete reduction of O₂ to water is a multistep reaction, which involves splitting of the O-O bond and transfer of 4e. The ORR complexity hampers the development of synthetic cost-efficient electrocatalysts and the massive commercialization of PEM fuel cells. Co complexes involving the Co phthalocyanine (CoPc) have been extensively studied as ORR electrocatalysts. Among these complexes, vitamin B₁₂ (a naturally occurring molecule) has attracted attention because, instead of catalysing the 2e reduction of O₂ to H₂O₂ as CoPc, it promotes the 4e reduction to H₂O. In this work, we study a CoPc axially coordinated to pyridine, which is anchored to a metallic carbon nanotube (CoPc-Py-CNT), biomimicking the vitamin B₁₂ active site. Density functional theory calculations are used to characterize the O₂ adsorption and dissociation on the metal center of CoPc-Py-CNT, whereas the synthetized catalyst was characterized by EPR and XPS spectroscopy, revealing insights into the mechanism of action of Co penta-coordinated complexes.   

Robust and lifelike order in non-equilibrium pseudo-crystals

Crystal structures are important in many fields of material science, however, such structures often exhibit inflexible order. Future technologies will require the creation of smart-materials that can change their properties on demand and exhibit multi-functional responses. Recent research has shown that order can emerge in structures that are maintained far-from-thermodynamic equilibrium through a process of self-organization. Self-organization is frequently observed in living systems, with artificially self-organized structures also displaying life-like behavior, such as with the abilities to self-heal with damage and adapt with their surroundings. In this talk, I will report a new form of non-equilibrium material that is characterized by robust pseudo-crystalline ordering. This order is sparsely periodic, with integer spacings between neighboring elements. Here, the particle-particle interactions that underlie collective ordering are mediated by wave scattering, which is externally tunable by varying the wavelength of a coherent drive. The sparse ordering allows our system to be exceedingly robust to both large mechanical perturbations and a changing environment.   

3D active nematics composed of microtubules and fd virus

We doped a passive 3D nematic liquid crystal composed of elongated fd virus with Microtubules (MTs) and clusters of Molecular motors that can slide the MTs passed one another. This composite material assembles into a three dimensional active liquid crystal. We are currently looking at the structure and the dynamics resulting from the active sliding of the MTs and how the interplay between activity, nematic elasticity and confinement affect the dynamical state of the complex fluid.   

Understanding Large Temperature Responsivity of Plant-Inspired Polymer

The electrical resistance of higher plants has been reported to be exponentially correlated with the ambient temperature. Their large temperature sensitivity is attributed to the pectin-calcium-ion complex in the plant cell wall. Our group has fabricated flexible transparent films, containing pectin and calcium chloride, and measured two-order of magnitude higher response than the best flexible temperature sensing films, between 0 to 45 Celsius. The potential application of this material is broad, ranging from temperature-sensing artificial skin to micro-bolometers. However, the physics of this high temperature sensitivity is still unknown. In order to understand the molecular mechanism of this behavior and to optimize its properties, we performed spectroscopic experiments and computational modeling on various pectin-based systems. Results explain how ion-pectin interactions change with temperature, and shed some light on the mechanisms behind the record-high temperature responsivity.   

The Emergence of Super-Swelled Lipid Bicontinuous Single-Crystals

Lipid bicontinuous cubic structures are arguably one of the most fascinating structures that lipid membranes are able to adopt. Their function in nature is far from unveiled but they are known, for example, to template the large unit cells (300 nm) of chitin that assign the structural color of butterfly wings. However, in the lab, lipid bicontinuous unit cells do not typically exceed 10-20 nm. We will discuss how processing of charged ternary systems out-of-equilibrium leads to super-swelled bicontinuous cubic phases. Unexpectedly, processing conditions also dictate mesophase ordering. X-ray and electron microscopy diffraction data reveals that lipid bicontinuous cubic phases can develop as perfect single-crystals of unit cell dimensions exceeding 60 nm.
  

3D axisymmetric shapes for colloidal membranes

We investigate the formation of catenoid-like colloidal membranes, fluid monolayers consisting of rod-like viruses held together by the depletion interaction. These catenoid-like membranes arise from the merging of two saddle-shaped membranes of smaller area that have been doped with shorter rods. Measurements show that the resulting structures are not true minimal surfaces but deviate slightly due to properties of the exposed edges. Using both an effective theory to describe the liquid crystal degrees of freedom by geometric properties of the edge and an explicit treatment of the liquid crystal terms, we calculate the possible axisymmetric equilibrium shapes and a phase diagram of the saddle-catenoid transition.   

Self-assembly of model protocell membranes

Fatty acids, encountered commonly as soap, form micelles at pH values above their pKa and neat oil below their pKa. At a pH range about their pKa, however, they form bilayers much like the phospholipid membranes that comprise the cell membranes of life on Earth. Because of their chemical simplicity and easy self-assembly, they are of great interest as components of the membranes of potential protocells on early Earth.

We find that assembling fatty acid vesicles from micelles gives almost 100% yield of unilamellar vesicles under certain circumstances, and discuss the conditions under which unilamellar fatty acid vesicle formation is thermodynamically favoured.

We also find that creating fatty acid vesicles from neat-oil in the presence of extensive surface area features (e.g. colloidal particles, porous materials) strongly biases the population towards giant oligolamellar vesicles. This has implications for those studying the origins of life because it suggests that giant uni- and oligo-lamellar vesicles may have formed in abundance in settings where sediment and porous substrates were likely present.