Physics : This is caused by what force?

as hot air rises and the shower curtain does not touch the bottom of the shower, cold air rushes in the bottom to replace the rising hot air, this rush of cold air draws the curtain as well. You will notice if you separate the curtain from the wall slightly , the curtain is no longer drawn in . this is because there is no pressure difference created between the top of the shower and the bottom of the shower; therefore no net air flow results . The water droplets are not relevant it is the hot and cold air that is the cause of this phenomenon .