X-ray-induced changes in wettability

Optimizing pink-beam fast X-ray microtomography for multiphase flow in 3D porous media

A fast pink-beam X-ray microtomography methodology was developed at the GSECARS 13-BMD beamline at the Advanced Photon Source to study multiphase flow in porous media. The white beam X-ray distribution of the Advanced Photon Source is modified using a 1-mm copper filter and the beam is reflected off a platinum mirror angled at 1.5 mrad, resulting in a pink beam with X-ray intensities predominately in the range of 40-60 keV. Bubble formation in the wetting phase and wettability alteration of the solid phase from x-ray exposure can be a problem with high flux and high energy beams, but the suggested pink-beam configuration mitigates these effects. With a 14-second acquisition time for capturing a complete dataset, the evolving fluid-fronts of nonequilibrium three-dimensional multiphase flow can be studied in real-time and the images contain adequate image contrast and quality to measure important multiphase quantities such as contact angles and interfacial areas. LAY DESCRIPTION: Understanding how fluids are transported through porous materials is pertinent to many important societal processes in the environment (e.g. groundwater flow for drinking water) and industry (e.g. drying of industrial materials such as pulp and paper). To develop accurate models and theories of this fluid transportation, experiments need to track fluids in 3-dimensions quickly. This is difficult to do as most materials are opaque and therefore cameras cannot capture fluid movement directly. But, with the help of x-rays, scientists can track fluids in 3D using an imaging technique called x-ray microtomography (μCT). Standard μCT takes about 15 minutes for one image which can produce blurry images if fluids are flowing quickly through the material. We present a technique, fast μCT, which uses a larger spectrum of x-rays than the standard technique and acquires a 3D image in 14 seconds. With the large amount of x-rays utilized in this technique, bubbles can start to form in the fluids from x-ray exposure. We optimized the utilized x-ray spectrum to limit bubble formation while still achieving a rapid 3D image acquisition that has adequate image quality and contrast. With this technique, scientists can study fluid transport in 3D porous materials in near real-time for the improvement of models used to ensure public and environmental health.

X-ray irradiation-induced reversible wettability modification of titanium NRAs

The reversible transition between hydrophilicity and hydrophobicity in Ti NRAs induced by alternating X-ray irradiation and ethanol immersion was revealed.

How does an air film evolve into a bubble during drop impact?

When a liquid drop impacts a solid surface, air is generally entrapped underneath. Using ultrafast x-ray phase-contrast imaging, we directly visualized the profile of an entrapped air film and its evolution into a bubble during drop impact. We identified a complicated evolution process that consists of three stages: inertial retraction of the air film, contraction of the top air surface into a bubble, and pinch-off of a daughter droplet inside the bubble. Energy transfer during retraction drives the contraction and pinch-off of a daughter droplet. The wettability of the solid surface affects the detachment of the bubble, suggesting a method for bubble elimination in many drop-impact applications.

pH-controllable water permeation through a nanostructured copper mesh film

Water permeation is an important issue in both fundamental research and industrial applications. In this work, we report a novel strategy to realize the controllable water permeation on the mixed thiol (containing both alkyl and carboxylic acid groups) modified nanostructured copper mesh films. For acidic and neutral water, the film is superhydrophobic, and the water cannot permeate the film because of the large negative capillary effect resulting from the nanostructures. For basic water, the film shows superhydrophilic property, and thus the water can permeate the film easily. The permeation process of water can be controlled just by simply altering the water pH. A detailed investigation indicates that nanostructures on the substrate and the appropriate size of the microscale mesh pores can enhance not only the static wettability but also the dynamic properties. The excellent controllability of water permeation is ascribed to the combined effect of the chemical variation of the carboxylic acid group and the microstructures on the substrate. This work may provide interesting insight into the new applications that are relevant to the surface wettability, such as filtration, microfluidic device, and some separation systems.

Redox-switchable superhydrophobic silver composite

Unique packaging of Ag(2)O on the surface of polycrystalline AgCl allows fabrication of a new useful, superhydrophobic composite material. This pure inorganic material with surface porosity of submicrometer aperture size fabricates air pockets, which make the composite material superhydrophobic. The new material behaves like lotus leaves, butterfly wings, or water strider's leg in relation to superhydrophobicity. Visible light induces photoreduction of solid Ag(2)O surface layer and generates Ag(0), making the composite surface superhydrophilic. Reoxidation of Ag(0) on the composite surface gives back the hydrophobicity that represents the redox-switchable wetting property of the material.

Cellulose-based liquid crystalline photoresponsive films with tunable surface wettability

We report on a new type of liquid crystalline cellulosic films with light controllable reversible wettability. The films are prepared from a thermotropic cellulose derivative functionalized with azo-containing groups. These groups exhibit dynamic changes in interfacial properties in response to UV irradiation. The UV irradiation induces trans-to-cis isomerization in the azobenzene moiety, which causes a conformational change in the upper molecular layers of the thin films. These changes originate a hydrophobic to comparatively hydrophilic transformation of the surface. The reversible wettability of the surface results from the cis/trans photo and thermal isomerization. The UV-vis absorption spectra, as well as contact angle measurements with UV irradiation, clearly support the understanding of the phenomenon. This type of surface design enables the amplification of molecular level conformational transitions to macroscopic changes in interface properties using the means of isomerism. This opens new opportunities in surface engineering using eco-friendly cellulose manipulation.