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1. Application of Digital Image Correlation to the Local Strain Analysis of Mouse Aortas: Novel Method to Create Speckle Pattern

   Du, Liya; Lane, Brooks A; Eberth, John F; Lessner, Susan M (June 2019, Proceedings of the 2019 Summer Biomechanics, Bioengineering, and Biotransport Conference)

   Digital image correlation (DIC) is a non-destructive and non-contact optical technique to measure deformation and strain of materials. The method is based on optically tracking the displacements of a speckle pattern created on the material surface. In the case of soft tissues such as mouse aorta, there are several advantages to using DIC since it can provide local, rather than global, deformations and it is suitable for large strain measurements, typical of soft tissues taken to failure [1] [2]. For the optimal use of DIC, several requirements should be met for speckle patterning: 1) randomness, 2) high contrast, 3) appropriate size of speckle in the field of view (3-5 pixels), and 4) firm attachment of speckle to specimen during deformation. In previous DIC studies of soft tissues, the methods employed to create a speckle pattern include the use of an airbrush to spray dye or paint on the specimen, or coating the sample with toner powder. However, biological samples must be partially dehydrated before applying paint which may affect the mechanical properties of the specimen, and toner powder is too hydrophobic to adhere well on specimens when submerged in aqueous solution during mechanical testing. In addition, it is difficult to evenly distribute paint or toner powder on the surface of a hydrated biological specimen [2]. Therefore, a novel method utilizing colloidal gold particles to create a speckle pattern on mouse aorta is proposed in this work. 

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2. Scalable and adaptable two-ligand co-solvent transfer methodology for gold bipyramids to organic solvents

   https://doi.org/10.1039/D4NA00527A  

   Coplan, Caitlin D; Watkins, Nicolas E; Lin, Xiao-Min; Schaller, Richard D (September 2024, Nanoscale Advances)

   Large and faceted nanoparticles, such as gold bipyramids, presently require synthesis using alkyl ammonium halide ligands in aqueous conditions to stabilize the structure, which impedes subsequent transfer and suspension of such nanoparticles in low polarity solvents despite success with few nanometer gold nanoparticles of shapes such as spheres. Phase transfer methodologies present a feasible avenue to maintain colloidal stability of suspensions and move high surface energy particles into organic solvent environments. Here, we present a method to yield stable suspensions of gold bipyramids in low-polarity solvents, including methanol, dimethylformamide, chloroform, and toluene, through the requisite combination of two capping agents and the presence of a co-solvent. By utilizing PEG-SH functionalization for stability, dodecanethiol (DDT) as the organic-soluble capping agent, and methanol to aid in the phase transfer, gold bipyramids with a wide-range of aspect ratios and sizes can be transferred between water and chloroform readily and maintain colloidal stability. Subsequent transfer to various organic and low-polarity solvents is then demonstrated for the first time. 

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3. Programmable Crowding and Tunable Phases in a Binary Mixture of Colloidal Particles under Light-Driven Thermal Convection

   https://doi.org/10.1021/acs.jpcb.4c02301  

   Lopez-Ceja, Jose; Flores, Vanessa; Juliano, Shirlaine; Machler, Sean; Smith, Stephen; Mansingh, Gargi; Shen, Meng; Tanjeem, Nabila (September 2024, The Journal of Physical Chemistry B)

   We employ photothermally driven self-assembly of colloidal particles to design microscopic structures with programmable size and tunable order. The experimental system is based on a binary mixture of “plasmonic heater” gold nanoparticles and “assembly building block” microparticles. Photothermal heating of the gold nanoparticles under visible light causes a natural convection flow that efficiently assembles the microscale building block particles (diameter 1–10 μm) into a monolayer. We identify the onset of active Brownian motion of colloidal particles under this convective flow by varying the conditions of light intensity, gold nanoparticle concentration, and sample height. We realize a crowded assembly of microparticles around the center of illumination and show that the size of the particle crowd can be programmed using patterned light illumination. In a binary mixture of gold nanoparticles and polystyrene microparticles, we demonstrate the formation of rapid and large-scale crystalline monolayers, covering an area of 0.88 mm2 within 10 min. We find that the structural order of the assembly can be tuned by varying the surface charge of the nanoparticles and the size of the microparticles, giving rise to the formation of different phases–colloidal crystals, crowds, and gels. Using Monte Carlo simulations, we explain how the phases emerge from the interplay between hydrodynamic and electrostatic interactions, as well as the assembly kinetics. Our study demonstrates the promise of self-assembly with programmable shapes and structural order under nonequilibrium conditions using an accessible setup comprising only binary mixtures and LED light. 

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4. Material-dependent performance of fuel-free, light-activated, self-propelling colloids

   https://doi.org/10.1039/d0cc00063a  

   Leeth Holterhoff, Andrew; Girgis, Victoria; Gibbs, John G. (April 2020, Chemical Communications)

   Self-propelling, light-activated colloidal particles can be actuated in water alone. Here we study the effect of adding different amounts of a gold/palladium alloy to titanium dioxide-based, active colloids. We observe a correlation between alloy-thickness and the average speed of the particles, and we discover an intermediate thickness leads to the highest activity for this system. We argue that a non-continuous thin-film of the co-catalyst improves the efficiency of water-splitting at the surface of the particles, and in-turn, the performance of “fuel-free” self-propulsion. 

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5. Engineering the Dynamics of Active Colloids by Targeted Design of Metal–Semiconductor Heterojunctions

   https://doi.org/10.1002/admi.201801894  

   Gibbs, John_G; Sarkar, Sumant; Leeth_Holterhoff, Andrew; Li, Mingyang; Castañeda, John; Toller, Justin (February 2019, Advanced Materials Interfaces)

   Abstract Self‐propelled colloids are primed to become scaled up, nano‐ and microscale inorganic analogues of molecular motors and machines. In order to advance toward the ambitious goal of employing such active particles to form genuine man‐made small scale machinery, a significantly diversified library of particle types, capable of a wide range of motive behaviors, must be available. Here, it is shown that the dynamics of photoactivated, self‐phoretic particles can be engineered by targeted design of metal–semiconductor heterojunctions. This effect is demonstrated with three different microswimmers consisting of an elongated semiconducting tail made from anatase titanium dioxide; all three of which would otherwise be identical absent vapor‐deposited coatings of gold at different locations on the tails. The specific location of the heterojunction determines the swimming behavior for each type. Although here only one shape and material combination is focused upon, engineering active particles with site‐specific metal–semiconductor heterojunctions is a general technique for achieving desired kinematic behavior in active colloidal matter. 

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