The effect of interfacial morphology on wetting of graphite by molten silver at high temperature

The wetting of graphite by molten silver was investigated at the temperature range from 1000 to 1350 °C using the sessile drop method coupled with a video-enhanced image digitization technology and a best-fitting algorithm. At the temperature range from 1100 to 1175 °C the contact angle θ was observed suddenly increasing and then decreasing, repeatedly. A similar behavior was not found in the comparison test performed on wetting of refractory (ZrO₂) by molten silver at the same range of temperature. A surface analysis of the graphite used in the measurements was performed, showing a rings-shaped surface morphology. The correlation between the wetting diameters, the ring diameters and the observed drop contractions was then brought out. The measured contact angle decreases while a “trough” forms at the three phases contact line. At the observed θ sudden increase, the drop is actually “jumping out” from the trough, since the rate of the formation of the trough is lower than the rate at which the drop changes its shape. The process then starts again, with the formation of a new trough. The measured θ is obviously affected by the trough geometry.