Contact angle measurement on porous substrates: Effect of liquid absorption and drop size

The contact angle Θ measured on rough, absorbing substrates is known to decrease with proceeding imbibition of the drop. We demonstrate that the impact of three absorption related parameters (absorption rate, drop size and residence time) on the measured contact angle is comprehensively captured by the relative absorbed drop volume, i.e. the percentage of the drop volume penetrated into the substrate. We analyzed a wide range of drop volumes (30 pl to 4 μl, spanning a factor 105) on substrates with very different liquid absorption rates (6–710 ml m−2s−1). Also the effect of liquid surface tension (26.8–73 mN m−1) and the effect of evaporation have been evaluated. Analyzing the development of drop volume and contact angle over time one can identify the regimes of (1) surface roughness filling and (2) bulk penetration. Independent of drop size, absorption rate and drop residence time the contact angle decreases between ~0. 5° to ~1. 2° for each percentage point of drop volume penetrated into the substrate. Thus, the relative amount of absorbed drop volume fully captures the combined effect of drop size, contact time and substrate absorption rate on the contact angle measurement, it can be used to estimate the effect of liquid penetration on the measured contact angle. The effect of evaporation was negligible for most water based liquids. In conclusion, contact angle values measured on absorbing substrates can only be compared for the same percentage of absorbed drop volume, e.g. when 30% of the drop volume has been absorbed.