Influence of tube wettability on water contact angle of powders determined by capillary rise

• Water contact angle of powder (θw) was assessed by Washburn capillary rise (WCR).
• Packing tube wettability affected θw determined by WCR.
• A hydrophobic tube increased θw of a highly wettable powder.
• The effect of tube wettability on θw was more dramatic by using a smaller tube.

During a recent literature review, we have noticed that researchers used tubes made of materials having a wide range of wettability, including plastics and metals, to pack powders for wettability determination using the Washburn capillary rise (WCR) method. Most studies applying WCR have focused on the effects of powder properties and their packing methods on the contact angle, while neglecting the role of packing tube. Although the ratio of packed powder surface area to tube surface area is large, we believe that tube wettability could have a significant effect on powder wettability determined using the WCR method. In this paper, we report that when water was penetrating the packed powder inside hydrophobic tubes, the water front along the tube wall was severely pinned, and in some cases, it was completely pinned at the support/packing/tube wall interface until the water had penetrated all the way up the packed powder. Using hydrophobic tubes limited the water rise to the powder region, which resulted in contact angles that were larger (up to ∼37°) than ones obtained using hydrophilic tubes, especially when the tube size was small.

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