Short timescale wetting and penetration on porous sheets measured with ultrasound, direct absorption and contact angle

Abstract

In this study the short timescale penetration and spreading of liquids on porous sheets is investigated. Three measurement techniques are evaluated: ultrasonic liquid penetration measurement (ULP), contact angle measurement (CA) and scanning absorptiometry (SA). With each of these techniques liquid penetration as well as surface wetting can be measured. A quantitative comparison between the methods is carried out. For our studies we are using model liquids with tuneable surface tension, viscosity and surface energy which are the governing parameters for pore flow according to the Lucas-Washburn equation.Scanning absorptiometry turns out to be an adequate tool for direct measurement for liquid penetration.Ultrasonic liquid penetration showed a stable correlation (R 2 ¼ 0.70) to SA and thus also gives a suitable indication on the liquid penetration behaviour. Absorption of individual microliter drops measured in the CA instrument showed different results than the other two measurements. For characterisation of the wetting behaviour the measurement techniques gave substantially different results. We thus conclude that ULP and SA do not capture the wetting behaviour of liquids on paper in the same way as conventional contact angle measurement, it is unclear if their results are meaningful. Finally we are proposing two parameters indicating a combination of liquid penetration and wetting, the slope of the contact angle over time dq/dt and a contact angle calculated from SA. These two parameters are moderately correlated, supporting the idea that they are indeed capturing a combination of liquid penetration and wetting. While our investigations are restricted to paper, we believe that the methods investigated here are generally applicable to study liquid absorption in thin porous media like microfluidic paper based analytical devices, thin porous storage media, membranes and the like. Our findings are highlighting the importance to have a match in timescale (time for penetration and wetting) and size scale (liquid amount supplied) between the testing method and the actual use case of the material, when analyzing wetting and penetration on porous materials.

Influence of polysiloxane microspheres on hydrophobicity, structure and mechanical properties of paper materials

“…The faster the liquid penetrates into the paper, the faster the ultrasonic signal decreases. Therefore the slope of the curve beyond the Max (I) value can be considered as the penetration speed (DI/Dt) (Krainer and Hirn 2018;Waldner and Hirn 2020). In the presented research, the time base for penetration speed calculations was 3 s. Results are presented in Table 6; Fig.…”

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

“…The parameter used most often for this type of curve is the time until the maximum intensity is reached (t max ) [3][4][5]8,[11][12][13][14][15][16]23,35], which often has been interpreted as the wetting time. As we pointed out, it is rather related to the onset of fiber swelling.…”

“…• A30/A60 = area above the curve after maximum until 30/60 s measurement time [3,4,23]; • Slope after maximum [8,11,12]; • T95/T60 = time until ultrasound intensity declined to 95/60 percent (after maximum) [5,13,21,22].…”

“…Since then, ultrasonic liquid penetration (ULP) measurements have been applied to study different aspects of interactions between liquids and porous, sheet like materials (paper, membranes, and thin films). Things investigated were for example network hydrophobization (sizing) [3][4][5][6], kinetics of liquid absorption (wetting and penetration) [7][8][9][10], evaluating water/grease barrier properties [11][12][13][14], as well as predicting print quality [15][16][17][18] and gluing strength [19]. The method has even been used to characterize the structure of the paper substrate, e.g., surface pores and roughness [20][21][22][23].…”

“…Without this understanding, interpretation of results is difficult. For instance, the time to the maximum intensity has been interpreted as the wetting time (e.g., [3,5,15]), but no correlation with wetting measured via contact angle measurements could be found [8]. Therefore, the aim of this paper is to clarify how these mechanisms influence ultrasound transmission, which of the mechanisms is dominant in certain circumstances and finally, to provide a guideline for interpretation of the result curves.…”