What phenomenon describes the formation and collapse of a gas pocket on an impeller's blade?

The phenomenon that describes the formation and collapse of a gas pocket on an impeller's blade is cavitation. Cavitation occurs when the pressure in a liquid drops below its vapor pressure, leading to the formation of vapor bubbles. When these bubbles are exposed to higher pressure regions as they move with the impeller, they can collapse violently, causing shock waves. This collapse can lead to significant damage to the impeller and other components of the pump. Cavitation is critical to understand in utilities and maintenance work because it can impair the function of equipment, reduce efficiency, and lead to costly repairs.

Other options, while related to fluid behavior, do not accurately define this specific phenomenon. Vibration refers to oscillations of a system, friction pertains to the resistance to motion, and entrainment relates to the incorporation of one fluid into another, neither of which captures the dynamics of gas pocket formation and collapse associated with cavitation.