A method for producing a composition comprising nano-bubbles dispersed in a liquid carrier using an apparatus, the apparatus comprising (a) an elongate housing comprising a first end and a second end, the housing defining a liquid inlet, a liquid outlet, and an interior cavity adapted for receiving the liquid carrier from a liquid source; and (b) a gas-permeable member at least partially disposed within the interior cavity of the housing, the gas-permeable member comprising an open end adapted for receiving a pressurized gas from a gas source, a closed end, and a porous sidewall extending between the open and closed ends having a mean pore size no greater than 1.0 μm, the gas-permeable member defining an inner surface, an outer surface, and a lumen, the liquid inlet of the housing being arranged to introduce the liquid carrier from the liquid source into the interior cavity of the housing at an angle that is generally orthogonal to the direction of liquid flow through the housing, the housing and gas-permeable member being configured such that pressurized gas introduced into the lumen of the gas-permeable member is forced through the porous sidewall of the gas-permeable member and onto the outer surface of the gas permeable member in the form of nano-bubbles as the liquid carrier from the liquid source flows parallel to the outer surface of the gas-permeable member from the liquid inlet to the liquid outlet, forming a composition comprising the liquid carrier and the nano-bubbles dispersed therein; said method comprising: introducing the liquid carrier from the liquid source into the interior cavity of the housing through the liquid inlet of the housing at a flow rate that creates turbulent flow at the outer surface of the gas-permeable member; and introducing the pressurized gas from the gas source into the lumen of the gas-permeable member at a gas pressure selected such that the pressure within the lumen is greater than the pressure in the interior cavity of the housing, thereby forcing gas through the porous sidewall and forming nano-bubbles on the outer surface of the gas-permeable member, wherein the liquid carrier flowing parallel to the outer surface of the gas-permeable member from the liquid inlet to the liquid outlet removes nano-bubbles from the outer surface of the gas-permeable member to form a composition comprising the liquid carrier and the nano-bubbles dispersed therein. (a) an elongate housing comprising a first end and a second end, the housing defining a liquid inlet, a liquid outlet, and an interior cavity adapted for receiving the liquid carrier from a liquid source; and (b) a gas-permeable member at least partially disposed within the interior cavity of the housing, the gas-permeable member comprising an open end adapted for receiving a pressurized gas from a gas source, a closed end, and a porous sidewall extending between the open and closed ends having a mean pore size no greater than 1.0 μm, the gas-permeable member defining an inner surface, an outer surface, and a lumen, the liquid inlet of the housing being arranged to introduce the liquid carrier from the liquid source into the interior cavity of the housing at an angle that is generally orthogonal to the direction of liquid flow through the housing, the housing and gas-permeable member being configured such that pressurized gas introduced into the lumen of the gas-permeable member is forced through the porous sidewall of the gas-permeable member and onto the outer surface of the gas permeable member in the form of nano-bubbles as the liquid carrier from the liquid source flows parallel to the outer surface of the gas-permeable member from the liquid inlet to the liquid outlet, forming a composition comprising the liquid carrier and the nano-bubbles dispersed therein; introducing the liquid carrier from the liquid source into the interior cavity of the housing through the liquid inlet of the housing at a flow rate that creates turbulent flow at the outer surface of the gas-permeable member; and introducing the pressurized gas from the gas source into the lumen of the gas-permeable member at a gas pressure selected such that the pressure within the lumen is greater than the pressure in the interior cavity of the housing, thereby forcing gas through the porous sidewall and forming nano-bubbles on the outer surface of the gas-permeable member, wherein the liquid carrier flowing parallel to the outer surface of the gas-permeable member from the liquid inlet to the liquid outlet removes nano-bubbles from the outer surface of the gas-permeable member to form a composition comprising the liquid carrier and the nano-bubbles dispersed therein.