Droplets wetting on filament rails: surface energy and morphology transition.

The morphology of liquid droplets wetting on filaments depends on the filament configuration, droplet volume, and contact angle. A stable morphology is the one that minimizes the potential energy of the droplet-filament system, while morphology transition may happen when an intermediate state exists which corresponds to a higher potential energy. This paper aims to explore such morphology transition of droplet wetting on filament rails made of two parallel identical microfilaments. Detailed numerical simulations were performed to extract the surface energy of the droplet-filament system at varying filament spacings, droplet volumes, and contact angles. Critical conditions of the morphology transition between two symmetrical wetting morphologies (i.e., liquid droplet bridge and barrel-shaped droplet) were determined. A family of characteristic curves in terms of the dimensionless droplet volume vs the filament spacing at varying contact angles was obtained, which can be used as a universal law to govern the morphology transition for such droplet-filament rail systems. The results and concepts presented in this work can be extended to broad wetting systems and utilized for the analysis and design of microfluidic devices and testers based on droplet-filament systems.

The Aqualon SLT: a novel device for measuring hair stiffness and lubricity.

The ability to quantify hair property changes in response to treatment is essential to the successful development of new formulations and benefiting agents. In the attempt to expand the toolbox of hair tress testing tools, we developed a device that allows hair scientists to measure hair tress changes in stiffness and lubricity. The tool is based on a system of pins mounted on free rotating bearings and is operated in two modes: rotating and stationary. The hair attributes are measured by threading a hair tress through the pin assembly and measuring the total work of pulling through in rotational and stationary modes (the latter mode is obtained by immobilizing pins by a retaining plate). The data thus obtained is de-convoluted into the work of apparent stiffness (rotational mode) and the work of the friction-on-pins or lubricity (stationary mode minus the apparent stiffness). The data can be further reduced to produce an apparent friction coefficient defined as a ratio of the apparent lubricity to the apparent stiffness. This work demonstrates the utility of the parameters measured by the Aqualon SLT and illustrates how the device can be used to predict and understand the impacts of various hair treatments.