* Threshold velocity for fluidization: This refers to the minimum velocity of a fluid required to suspend solid particles in a fluidized bed. In this context, fine, light particles with low density would have the lowest threshold velocity.
* Threshold velocity for erosion: This refers to the minimum velocity of a fluid needed to erode a solid surface. In this context, soft, easily eroded materials like loose soil or sand would have the lowest threshold velocity.
* Threshold velocity for drag: This could refer to the minimum velocity at which an object experiences significant drag force. In this context, objects with large surface areas and low mass would have the lowest threshold velocity.
Therefore, to answer your question properly, you need to specify the context in which you are referring to "threshold velocity". Please clarify the context so I can provide a more specific answer.
