Probing the dynamics of turbid colloidal suspensions using differential dynamic microscopy.

Few techniques can reliably measure the dynamics of colloidal suspensions or other soft materials over a wide range of turbidities. Here we systematically investigate the capability of Differential Dynamic Microscopy (DDM) to characterise particle dynamics in turbid colloidal suspensions based on brightfield optical microscopy. We measure the Intermediate Scattering Function (ISF) of polystyrene microspheres suspended in water over a range of concentrations, turbidities, and up to 4 orders of magnitude in time-scales. These DDM results are compared to data obtained from both Dynamic Light Scattering (DLS) and Two-colour Dynamic Light Scattering (TCDLS). The latter allows for suppression of multiple scattering for moderately turbid suspensions. We find that DDM can obtain reliable diffusion coefficients at up to 10 and 1000 times higher particle concentrations than TCDLS and standard DLS, respectively. Additionally, we investigate the roles of the four length-scales relevant when imaging a suspension: the sample thickness L, the imaging depth z, the imaging depth of field DoF, and the photon mean free path . More detailed experiments and analysis reveal the appearance of a short-time process as turbidity is increased, which we associate with multiple scattering events within the imaging depth of the field. The long-time process corresponds to the particle dynamics from which particle-size can be estimated in the case of non-interacting particles. Finally, we provide a simple theoretical framework, ms-DDM, for turbid samples, which accounts for multiple scattering.

The effect of salt and particle concentration on the dynamic self-assembly of detonation nanodiamonds in water.

Detonation nanodiamonds (DNDs) are becoming increasingly important in science and technology with applications from drug delivery to tribology. DNDs are known to self-assemble into fractal-like aggregates in water, but their colloidal properties remain poorly understood. Here, the effect of salt and particle concentration on the size and shape of these aggregates is investigated using dynamic light scattering and small-angle X-ray scattering. Our results suggest the existence of two particle aggregate populations with diameters on the scale of 50 nm and 300 nm, respectively. The concentration of NaCl, in the range 0.005-1 mM, does not have a significant effect on the size or shape of the particle aggregates. The hydrodynamic radius of both aggregate populations decreases as the DND concentration increases from 0.01 to 2 mg mL-1. At the same time, the particle aggregates become denser and their overall shape changes from disk-like to rod-like with increasing DND concentration. We identify unexpected similarities between the aggregate structures observed for DNDs and those commonly observed for concentrated colloidal particles in high salt environments, described by classical colloid aggregation theories. Our results contribute to the fundamental understanding of the colloidal properties of DNDs and pave the way for the engineering of novel nanoparticle-based systems that make use of DNDs' unique colloidal properties for future applications.

Differential dynamic microscopy to measure the translational diffusion coefficient of nanorods.

We explore differential dynamic microscopy (DDM) as a low angle scattering technique to determine the translational diffusion coefficients of gold rod shaped nanoparticles. The method is tested using five differently sized nanorods, and compared with results obtained from polarized dynamic light scattering. For the rods studied here, the method of DDM may be a more robust technique as obtaining the translational diffusion coefficient is more straightforward. Results obtained from DDM are then used as an input to fitting depolarized dynamic light scattering data for the determination of the rotational diffusion coefficient. The measured diffusion coefficients are compared with theoretical predictions based on rod sizes.

A Gallium-Based Magnetocaloric Liquid Metal Ferrofluid.

We demonstrate a magnetocaloric ferrofluid based on a gadolinium saturated liquid metal matrix, using a gallium-based liquid metal alloy as the solvent and suspension medium. The material is liquid at room temperature, while exhibiting spontaneous magnetization and a large magnetocaloric effect. The magnetic properties were attributed to the formation of gadolinium nanoparticles suspended within the liquid gallium alloy, which acts as a reaction solvent during the nanoparticle synthesis. High nanoparticle weight fractions exceeding 2% could be suspended within the liquid metal matrix. The liquid metal ferrofluid shows promise for magnetocaloric cooling due to its high thermal conductivity and its liquid nature. Magnetic and thermoanalytic characterizations reveal that the developed material remains liquid within the temperature window required for domestic refrigeration purposes, which enables future fluidic magnetocaloric devices. Additionally, the observed formation of nanometer-sized metallic particles within the supersaturated liquid metal solution has general implications for chemical synthesis and provides a new synthetic pathway toward metallic nanoparticles based on highly reactive rare earth metals.

Photomodulation of bacterial growth and biofilm formation using carbohydrate-based surfactants.

Naturally occurring and synthetic carbohydrate amphiphiles have emerged as a promising class of antimicrobial and antiadhesive agents that act through a number of dynamic and often poorly understood mechanisms. In this paper, we provide the first report on the application of azobenzene trans-cis photoisomerization for effecting spatial and temporal control over bacterial growth and biofilm formation using carbohydrate-based surfactants. Photocontrollable surface tension studies and small angle neutron scattering (SANS) revealed the diverse geometries and dimensions of self-assemblies (micelles) made possible through variation of the head group and UV-visible light irradiation. Using these light-addressable amphiphiles, we demonstrate optical control over the antibacterial activity and formation of biofilms against multi-drug resistant (MDR) Pseudomonas aeruginosa, methicillin-resistant Staphylococcus aureus (MRSA) and Gram-negative Escherichia coli. To probe the mechanism of bioactivity further, we evaluated the impact of trans-cis photoisomerization in these surfactants on bacterial motility and revealed photomodulated enhancement in swarming motility in P. aeruginosa. These light-responsive amphiphiles should attract significant interest as a new class of antibacterial agents and as investigational tools for probing the complex mechanisms underpinning bacterial adhesion and biofilm formation.

Disposition and crystallization of saturated fatty acid in mixed micelles of relevance to lipid digestion.

During lipolysis of triglyceride by lipase, monoglyceride and fatty acids are produced which combine with gastrointestinal fluids to form self-assembled structures. These solubilize hydrophobic food components to promote their absorption. The aim of this study was a detailed understanding of structure formation from triglyceride digestion products with saturated short-, medium- and long chain fatty acids. Complementary characterization methods have been applied comprising small angle X-ray and neutron scattering ­ the latter involving the contrast matching technique using fully deuterated fatty acids ­ polarized and depolarized dynamic light scattering and cryogenic-transmission electron microscopy. Shape, size and solubilization capacity of the self-assembled structures was dependent on composition and lipid chain length. Crystallization of fatty acid was observed when the solubility limit in the mixed bile salt micelles was exceeded; however, increasing pH and temperature increased the fatty acid solubility. The results provide insight into structure formation and crystallization of incorporated lipolysis products; this is important for a detailed understanding of food structure and nutrition, as well as the rational design of lipid based drug delivery systems.