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Bubbles flow upwards because they are less dense than the liquid they are in.
Gravity does more than just pull on you, it accelerates you. In other words, if the ground were to suddenly drop away, you wouldn't simply fall at one speed, you would fall faster and faster as time went on. It is the acceleration of gravity that the bubbles feel, and they flow in the other direction. So you could make bubbles flow down if you could trick them into thinking gravity was pulling them up. They would respond by flowing down, the opposite direction.