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

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 (R2 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.