Ultrasonic Liquid Penetration Measurement in Thin Sheets-Physical Mechanisms and Interpretation

Liquid uptake in porous cellulose sheets studied with UFI-NMR: Penetration, swelling and air displacement

Liquid penetration in porous cellulosic materials is crucial in many technological fields. The complex geometry, small pore size, and often fast timescale of liquid uptake makes the process hard to capture. Effects such as swelling, vapor transport, film flow and water transport within cellulosic material makes transport deviate from well-known relations such as Lucas-Washburn and Darcy's Law. In this work it is demonstrated how Ultra-Fast Imaging NMR can be used to simultaneously monitor the liquid distribution and swelling during capillary uptake of water with a temporal- and spatial resolution of 10 ms and 14.5-18 μm respectively. The measurements show that in a cellulose fiber sheet, within the first 65 ms, liquid first penetrates the whole sheet before swelling takes place for another 30 s. Furthermore, it was observed that the liquid front traps 15 v% of air which is slowly replaced by water during the final stage of liquid uptake. Our method makes it possible to simultaneously quantify the concentration of all three phases (solid, liquid and air) within porous materials during processes exceeding 50 ms (5 times the temporal resolution). We hence believe that the proposed method should also be useful to study liquid penetration, or water diffusion, into other porous cellulosic materials like foams, membranes, nonwovens, textiles and films.

3D Printed Structures for Ultrasound Attenuation in Underwater Environment

In this work, open or closed air cavity (air bubble) inclusion structures are 3D printed via direct ink writing and fused deposition modeling methods utilizing materials of polydimethylsiloxane silicone or thermoplastic polyurethane, respectively, and these structures are examined for their attenuation capacity concerning ultrasonic waves in underwater environment. It is found that several factors, such as interstitial fencing layer, air cavity fraction, material interface interaction, and material property, are fundamental elements governing the overall attenuation performance. Hence, via 3D printing technique, which could conveniently manipulate structure's cavity volume fraction, such as via filament size and filament density on surface, structures with tunable attenuation could be designed. In addition, considering directions where ultrasound would encounter interfaces, that is, if the geometry could induce more interface interactions, such as triangular shape compared with simple square, it is possible to obtain immense attenuation enhancement, which does pave an additional approach for attenuation optimization via convoluted structural interface design that is exclusively tailored by additive manufacturing.

Modeling liquid penetration into porous materials based on substrate and liquid surface energies

HYPOTHESIS

The widely used Lucas-Washburn (LW) equation depends on the contact angle as the driving force for liquid penetration. However, the contact angle depends on both, the liquid and the substrate. It would be desirable to predict the penetration into porous materials, without the requirement to measure the solid-liquid interaction. Here, we propose a novel modeling approach for liquid penetration from mutually independent substrate- and liquid properties. For this purpose, the contact angle in the LW-equation is replaced by polar and dispersive surface energies, utilizing the theories of Owens-Wendt-Rabel-Kaelble (OWRK), Wu, or van Oss, Good, Chaudhury (vOGC).

EXPERIMENTS

The proposed modelling approach is validated exhaustively by measuring penetration speed for 96 substrate-liquid pairings and comparing the results to model predictions based on literature- and measured data.

FINDINGS

Liquid absorption is predicted very well (R² = 0.8-0.9) with all three approaches, spanning a wide range of penetration speed, substrate- and liquid surface energy, viscosity, and pore size. The models for liquid penetration without measurement of solid-liquid interaction (contact angle) performed well. Modeling calculations are entirely relying on physical data of the solid and the liquid phase (surface energies, viscosity and pore size), which can be measured or retrieved from databases.

Starch-Silane Structure and Its Influence on the Hydrophobic Properties of Paper

Starch is an inexpensive, easily accessible, and widespread natural polymer. Due to its properties and availability, this polysaccharide is an attractive precursor for sustainable products. Considering its exploitation in adhesives and coatings, the major drawback of starch is its high affinity towards water. This study aims to explain the influence of the silane-starch coating on the hydrophobic properties of paper. The analysis of the organosilicon modified starch properties showed an enhanced hydrophobic behavior, suggesting higher durability for the coatings. Molecules of silanes with short aliphatic carbon chains were easily embedded in the starch structure. Longer side chains of silanes were primarily localized on the surface of the starch structure. The best hydrophobic properties were obtained for the paper coated with the composition based on starch and methyltrimethoxysilane. This coating also improved the bursting resistance and compressive strength of the tested paper. A static contact angle higher than 115° was achieved. PDA analysis confirmed the examined material exhibited high barrier properties towards water. The results extend the knowledge of the interaction of silane compositions in the presence of starch.