Wetting via water condensation of hydrophobic grain surfaces as viewed by synchrotron-radiation computed tomography

Abstract The wetting of hydrophobic material via water droplet condensations by means of electronic microscopy and synchrotron computed tomography is investigated in this paper in order to better understand the mechanisms controlling the interaction of hydrophobic granular media and water. In literature so far the behavior has been investigated only by 2D means. In this study, sand and glass beads with similar grain size distribution were made hydrophobic via PFA-C6 cold plasma polymerization. A miniature cooling device based on the Peltier effect for in situ synchrotron computed tomography experiments controlled by a Raspberry Pi was designed and used to induce droplet condensation on the material’s surface during CT scanning. The tomograms allow to count droplets and measure their contact angles. Results show that the condensation of those small droplets feature contact angles usually greater than $$90^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mn>90</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:math> at the beginning of the process, but as time advances, those droplets coalesce, increasing in volume and decreasing in quantity, while their contact angles get progressively smaller. Although hydrophobic behavior and convex menisci were expected in the pendular and funicular regimes, the contact angles of these water structures were closer to $$60^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mn>60</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:math> at this phase, indicating hydrophilic tendencies. Similarly to wetting via water inflow, the hydrophobicity is lost after a specific degree of local saturation. Unfortunately, the results gathered in this study point in an unfavorable direction for the applications of the hydrophobization technique used here in the macroscale, given their low efficiency in repelling water after reaching the funicular state of the unsaturated regimen and the current discussions about their environmental implications.