a buoyant force pump is disclosed . a material which is buoyant relative to a non - solid material is used to cause a movable surface to move thereby causing the non - solid material to be pumped . the device can be used to move non - solid material in hazardous conditions or materials in biological reactors . the pumps can be used for mixing ingredients external to the pump by repeatedly moving materials into and out of the pump . the pumps can also be used to gently move materials from an inlet to an outlet .