4.6 Article

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

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ELSEVIER
DOI: 10.1016/j.colsurfa.2021.126503

Keywords

Contact angle; Absorbing substrate; Drop volume; Evaporation; Surface tension

Funding

  1. Austrian Federal Ministry for Digital and Economic Affairs
  2. National Foundation for Research Technology and Development
  3. Canon Production Printing
  4. Mondi
  5. Kelheim Fibres
  6. SIG Combibloc

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The study comprehensively captures the impact of absorption rate, drop size, and residence time on contact angle measurement through the concept of relative absorbed drop volume. Regardless of drop size, absorption rate, and residence time, the contact angle decreases with each percentage point of drop volume penetrated into the substrate.
The contact angle Theta 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 mu l, spanning a factor 10(5)) on substrates with very different liquid absorption rates (6-710 ml m(-2) s(-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 similar to 0. 5 degrees to similar to 1. 2 degrees 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.

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