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

**Carina Waldner 1,2 and Ulrich Hirn 1,2,\***


Received: 30 April 2020; Accepted: 10 June 2020; Published: 17 June 2020

**Abstract:** Ultrasonic liquid penetration (ULP) measurements of porous sheets have been applied for a variety of purposes ranging from determining liquid absorption dynamics to surface characterization of substrates. Interpretation of ULP results, however, is complex as the ultrasound signal can be affected by several mechanisms: (1) air being replaced by the liquid in the substrate pores, (2) air bubbles forming during penetration, and (3) structural changes of the substrate due to swelling of the substrate material. Analyzing tailored liquids and substrates in combination with contact angle measurements we are demonstrating that the characteristic shape of the ULP measurement curves can be interpreted in terms of the regime of liquid uptake. A fast and direct decline of the curve corresponds to capillary penetration, the slope of the curve indicates the penetration speed. A slow decline after a previous maximum in the signal can be related to diffusive liquid transport and swelling of the substrate material.

**Keywords:** liquid penetration; ultrasound transmission; capillary penetration; porous sheets
