Capillary-driven binding of thin triangular prisms at fluid interfaces.

We observe capillary-driven binding between thin, equilateral triangular prisms at a flat air-water interface. The edge length of the equilateral triangle face is 120 µm, and the thickness of the prism is varied between 2 and 20 µm. For thickness to length (T/L) ratios of 1/10 or less, pairs of triangles preferentially bind in either tip-to-tip or tip-to-midpoint edge configurations; for pairs of prisms of thickness T/L = 1/5, the tip of one triangle binds to any position along the other triangle's edge. The distinct binding configurations for small T/L ratios result from physical bowing of the prisms, a property that arises during their fabrication. When bowed prisms are placed at the air-water interface, two distinct polarity states arise: prisms either sit with their center of mass above or below the interface. The interface pins to the edge of the prism's concave face, resulting in an interface profile that is similar to that of a capillary hexapole, but with important deviations close to the prism that enable directed binding. We present corresponding theoretical and numerical analysis of the capillary interactions between these prisms and show how prism bowing and contact-line pinning yield a capillary hexapole-like interaction that results in the two sets of distinct, highly-directional binding events. Prisms of all T/L ratios self-assemble into space-spanning open networks; the results suggest design parameters for the fabrication of building blocks of ordered open structures such as the Kagome lattice.

Kinetics of colloidal deposition, assembly, and crystallization in steady electric fields.

We quantify and model the deposition and crystallization kinetics of initially dilute colloidal spheres due to application of a steady, direct current electric field in the thin gap between parallel electrodes. The system studied is poly(12-hydroxystearic acid) (PHSA)-stabilized poly(methyl methacrylate) (PMMA) spheres dispersed in a mixture of cyclohexylbromide (CHB), decalin, and a low concentration of the partially disassociating salt tetrabutylammonium chloride (TBAC). The temporal and spatial evolution of the colloidal volume fraction in the ∼1 mm gap between the electrodes is quantified under conditions of both deposition and relaxation by confocal laser scanning microscopy (CLSM). During deposition assembly, the spatial dependence of the colloid volume fraction approaches steady state at times between hundreds of minutes at the lowest electric field strength (as characterized by a Peclet number, Pe) and at tens of minutes at higher field strengths. During disassembly, the volume fraction relaxes nearly exponentially. The kinetics are modeled by adapting a treatment for sedimentation (Davis and Russel, Phys. Fluids A, 1989, 1, 82) to the case of steady electric fields. The model's predictions show good agreement with the measured kinetics at low Pe; however, agreement progressively deteriorates with increasing Pe. At low Pe the deposits are initially disordered. After an initial delay, 1D crystal growth propagates from the electrode surface at rates of several hundred nm min(-1). The sharp crystal boundary propagates as a characteristic of constant colloidal volume fraction, consistent with an equilibrium crystalline phase transition. The results inform operational ranges for devices that produce active colloidal matter by reversible assembly.

Towards an improved understanding of drug excipient interactions to enable rapid optimization of nanosuspension formulations.

Suspensions of drug nanoparticles known as nanosuspensions have emerged as a successful enabling formulation approach for poorly soluble drug candidates. These nanoparticles typically require stabilization with specific polymer or surfactant excipients to prevent aggregation from occurring. This study demonstrates the necessity of formulating drug nanosuspensions with amphiphilic excipients possessing long hydrophobic alkyl or polymer block chains to produce stable nanoparticles. 28 different excipients and excipient combinations at various loadings were screened across the 3 drug compounds and their effectiveness, as characterized by the lowest excipient loading needed to stabilize a monodisperse drug suspension, is quantified as a function of various excipient parameters such as molecular weight, HLB value, CMC, H-bond donors and acceptors, and the length of the hydrophobic alkyl chains and polymer blocks within their molecular structure. Traditional characterization parameters (molecular weight, HLB value, and CMC) fail to predict excipient effectiveness. The conformational flexibility and length of the hydrophobic regions of amphiphilic excipients appears to be critical for effectiveness. This hypothesis was supported by molecular modeling studies to better understand the interactions between the excipients with the drug nanoparticle surface.