Measuring liquid penetration in thin, porous sheets with ultrasound and drop absorption – scope and limitations

Measuring liquid absorption in thin, porous substrates is challenging, yet it is crucial for process performance and product properties in contexts like printing, textiles, or membranes. Drop absorption (DA) measurements and ultrasonic liquid penetration (ULP) measurements did not correlate in earlier work. This work explains the reasons for the poor agreement between the two methods and provides insight on the applicability and limita- tions of both. Two sets of systematically designed paper substrates and test liquids covering a wide range of penetration speeds were tested. DA and ULP correlated for most penetration speeds. DA results were more reliable in the low-absorbing region, while ULP measurements are especially suited to capture high absorption rates and detect smaller deviations in liquid penetration, which makes it well suited for industrial process control. Trustworthiness of the results has been found to be related to the contact angle of the liquid/substrate pair.