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<p>  </p> <p>In the practical applications of colloidal dispersions and suspensions, such as inks, paints, and food industry, the suspended particles must be stabilized, and remain well-dispersed for long times. Particles which are more dense than the suspending media may sediment rapidly, even with no agglomeration occurring, under many conditions of size and density difference. Then, a dispersant would be necessary for stabilization of particle suspensions against both agglomeration and sedimentation, while the suspensions should remain flowable in many applications. Moreover, when many aqueous suspension media may contain salts, the dispersant also needs to be an effective stabilizer against sedimentation under the specific salinity conditions of that application.</p> <p>DDAB (didodecyldimethylammonium bromide) , a cationic double-chain surfactant, forms lamellar liquid crystal phases when dispersed in water. It also easily forms aqueous vesicle dispersions (unilamellar closed particles with an internal solvent compartment) and liposomes (multilamellar vesicles, MLVs, or lamellar liquid crystallites) at relatively low DDAB weight fractions, <em>w</em>_D. To better understand the phase/dispersion behavior of DDAB and the corresponding optical properties, new analytical solutions of the spherical particles have been obtained for the light scattering theory in the Rayleigh (R) and the Rayleigh-Debye-Gans (RDG) regimes, for single and independent scattering. Moreover, the specific Rayleigh ratio <em>R</em>_q** and the specific turbidity <em>t</em>** were derived analytically for both scattering regimes. Spectroturbidimetry (ST) data at 25 °C for DDAB were compared to the <em>t</em>** predictions. <em>t</em>** data for DDAB vesicles are consistent with the RDG predictions, which are also used to estimate the vesicle sizes.</p> <p>For a better understanding of the effect of the preparation method and salinity on the formation of DDAB vesicles, spectroturbidimetry was used to measure the average radius of the unilamellar DDAB vesicles, which were prepared via two different methods in water and in NaBr salt solutions. The radius was ~24 nm after sonication (SS method) and ~74 nm after extrusion/ultrafiltration (SE method). The radii were larger when the vesicles were produced in 10 mM NaBr, ~65 nm for the SS method and ~280 nm for the SE method. The <em>t</em>** values of these vesicular dispersions increased with decreasing <em>w</em>_D values, until a constant value was reached at <em>w</em>_D*, which depends on the preparation method and the dispersion medium. The constant values of <em>t</em>** are indicative of single and independent scattering, and were used to estimate vesicle radii by solving the <em>t</em>** equations derived for the RDG regime. Estimates of the average distances between the vesicles and their corresponding Debye lengths were obtained to evaluate the importance of inter-vesicle electrostatic interactions, which could lead to dependent scattering at higher weight fractions.</p> <p>DDAB prepared with magnetic stirring of multilamellar liposomes, followed by ultrasonication to generate unilamellar vesicles, were found to have very high viscosities at very low shear stresses at DDAB weight fractions <em>w</em>D from 0.025 to 0.027. The vesicles had average diameters ranging from 68 to 80 nm, as previously determined from spectroturbidimetry. These vesicle dispersions stabilized suspensions of monodisperse spherical amorphous silica particles with diameters of <em>d</em>_sed = 454 nm, 691 nm, and 826 nm against sedimentation, at least for several weeks. Similar results were obtained for suspensions, in DDAB vesicle dispersions, of polydisperse, nonspherical, crystalline titania particles with sizes ranging from ca. 96 nm to 156 nm. At the relatively low values of <em>w</em>_D = 0.009 and 0.018, the effective viscosities,<em> h</em>eff, of the DDAB dispersions, determined from the sedimentation velocities, ranged from 1.35 to 1.87 cP and from 4.34 to 5.57 cP, respectively. At <em>w</em>_D = 0.027 for the silica particles with <em>d</em>_sed = 454 nm, or at <em>w</em>_D = 0.025 for all other particles considered, <em>h</em>_eff was essentially infinite, and each vesicle dispersion behaved as a gel at low shear stresses. At higher shear stresses, however, the dispersions were highly shear-thinning, and flowable in a capillary tube under gravity. This behavior is critical for the practical applications of such dispersions for paints and inkjet printing. To further understand the feasibility of the vesicle stabilization mechanism at various NaBr concentrations, <em>w</em>NaBr, the salinity effects on the stabilization of silica particles against sedimentation were also examined. It was found that at <em>w</em>_NaBr < 0.0020 and at <em>w</em>D > 0.060, the DDAB dispersion could stabilize silica particles against sedimentation for at least two weeks. The relationship of the phase and dispersion behavior of DDAB/aqueous NaBr solutions to their stabilizing effectiveness will be further studied.</p> <p>A first discovery of iridescent liquid-like aqueous vesicle dispersions formed from the DDAB is also reported. Although iridescence arises from some solid crystallites, thin films, and colloidal crystals, it had never been observed in systems that are liquid-like. Visual observations and ST at wavelengths of 350 nm to 700 nm were used to determine vesicle sizes and microstructure formation in dispersions for DDAB weight fractions <em>w</em>D between 0.020 to 0.030. The DDAB vesicle dispersions exhibited iridescent colors for <em>w</em>D = 0.023 to 0.027, due to the formation of “soft” crystallites formed by self-assembled vesicles. Effective vesicle radii from 30 to 60 nm were inferred from the ST measurements. The volume fractions of the vesicles <em>f</em>v and their effective volume fractions <em>f</em>v*, which account for the electrostatic double layers around a vesicle, were also estimated. The high values of <em>f</em>v* for the iridescent dispersions indicate that they contain neighboring vesicles with highly overlapping electrostatic double layers, even though their values of <em>f</em>v remain relatively low. Hence, strong electrostatic repulsive interactions arise between the vesicles. These interactions probably drive the formation of the “soft” crystallites, and thus the observed iridescence. Nevertheless, these “soft” crystallites, which could be easily broken up but were quick to reform, remain suspended. Consequently, these vesicle dispersions still flowed as a bulk dispersion with a high viscosity; the dispersion as a whole remained liquid-like or as a “liquid gem”, in contrast to what occurs to the other colloidal crystals made of rigid colloids. Beside their beautiful appearances, these DDAB vesicle dispersions also act as effective stabilizers of dense silica suspensions against sedimentation even at relatively low values of <em>w</em>D. </p>

Emulsions are commonly found both in nature and industry. Due to the complex nature of emulsion systems, their interfacial properties and stability are poorly understood, particularly the influence of droplet deformability on the colloid and interfacial behaviour. This study has highlighted the role of emulsion droplet cross-linking (deformability and penetrability) on droplet surface chemistry, droplet colloidal stability and adsorption at the droplet-water interface and provides insight into methods for enhancing the performance of emulsion formulations.

The surface force apparatus (SFA) has been an important technique for making direct force measurements and has contributed enormously to our understanding of colloidal interactions. The conventional SFA has been limited to measuring forces between solid surfaces, until recently when a modified SFA was developed at the Ian Wark Research Institute [1]. A fluid drop (mercury) is introduced into the apparatus which allows a range of deformable surfaces to be studied in the SFA. This project is an extension of this technique. Interactions between a mica sheet and a mercury drop are studied, including the modification of mercury with self-assembled monolayers (SAMs) of thiol surfactants, and the drop deformation due to non-equilibrium adsorption effects and hydrodynamic forces.

The surface force apparatus (SFA) has been an important technique for making direct force measurements and has contributed enormously to our understanding of colloidal interactions. The conventional SFA has been limited to measuring forces between solid surfaces, until recently when a modified SFA was developed at the Ian Wark Research Institute [1]. A fluid drop (mercury) is introduced into the apparatus which allows a range of deformable surfaces to be studied in the SFA. This project is an extension of this technique. Interactions between a mica sheet and a mercury drop are studied, including the modification of mercury with self-assembled monolayers (SAMs) of thiol surfactants, and the drop deformation due to non-equilibrium adsorption effects and hydrodynamic forces.

<p>Here presents the studies of polymerization kinetics and crosslinking efficiency of nm-resolution striped phases on surface, which depends on lengths of alkyl segments and headgroup chemistry. While fluorescence readouts offer the overall efficiencies of polymerization and crosslinking transfer, SPM measurements reveal molecular details accounting for reactivity differences. Additionally, this research also demonstrates the utilization of primary amines striped phases on soft materials, achieving post-functionalization and specific  adsorption of nanocrystals, highlighting the versatile applications of this nm-scale chemistry boundary.</p>

<p dir="ltr">There is a plethora of applications that require chemical patterning on the molecular scale. While the surface science community has made tremendous progress in achieving this level of control on hard, crystalline interfaces, significant challenges are associated with extending this progress to less “perfect” systems such as soft, amorphous interfaces. Applications ranging from soft robotics and wearable electronics to regenerative medicine often utilize polymeric materials such as polydimethylsiloxane (PDMS) and hydrogels. These materials have advantageous properties, including biocompatibility and mechanical tunability. Biological applications, for example, often require the display of functional groups with precise spatial resolution. Cellular behavior is dictated by biochemical and biophysical cues in the extracellular matrix; therefore, substrate properties, including stiffness and ligand density, must be independently tunable. Soft, polymeric materials are highly heterogenous with pore sizes ranging from 10 nm to 1 µm and hence, particularly difficult to pattern below the length scale of substrate heterogeneity. Furthermore, deconvolving mechanical properties such as elastic modulus from the density of surface-active functional groups is especially challenging, with softer materials typically corresponding is lower ligand densities. Additionally, many traditional surface science characterization and patterning methods are incompatible with soft interfaces (e.g. amorphous surface structure, low mechanical strength, hydrated environment). Recently, we have reported a method capable of achieving high-resolution chemical patterning of PDMS and hydrogels. Long studied within the scanning probe community, amphiphiles with long alkyl chains self-assemble into lying down stripe phases on highly ordered pyrolytic graphite (HOPG), generating 1-nm-wide stripes of functional headgroups between 5-nmwide stripes of exposed alkyl chains. Stripe phases of functional diacetylenes (DA) are photopolymerized, producing a polydiacetylene backbone that tethers together adjacent molecules, generating a PDA film on HOPG (sPDA). We have shown that PDA films on HOPG can be transferred to PDMS as well as polyacrylamide hydrogels. When PDMS is cured in contact with sPDAs, the PDA backbones can act as a site for hydrosilylation, the same reaction responsible for PDMS curing, covalently linking sPDAs to the PDMS mesh. Careful exfoliation reveals nm-scale functional patterns on the surface layer of PDMS. 10 Here, we examine the impact of PDMS structural components on the efficiency of interfacial reactions between sPDAs and the PDMS network. We also illustrate the impact of PDAfunctionalized PDMS on the adhesion and spreading behavior of C2C12 murine myoblasts.</p>