A foam-generating device, comprising: - a reservoir of a foaming liquid, - a compressed gas cylinder , - a first pipe out of the reservoir and passing through a pneumatic pump while being connected to suction and discharge ports of said pneumatic pump, - a second pipe out of the cylinder and passing through an expansion valve , the first pipe and the second pipe being joined at a junction ; - a pipe for transporting a mixture of the foaming liquid and the compressed gas extending between the junction and a foam ejection end ; - a mixer of the foaming liquid and the compressed gas, in which the foam is formed, and located on the pipe for transporting the mixture between the junction and the ejection end ; the second pipe comprising a bifurcation leading to a port for starting the pneumatic pump, characterised in that the mixer comprises : - a cylindrical wall open at the two axial ends and through which the mixture passes between said ends axial, - a cage formed in the cylindrical wall, - and at least one solid body which is free to move in the cage, the at least one solid body consisting of 1 to 40 balls, constructed of rigid materials, and in that the cage is 10 to 400 mm in length, 10 to 100 mm in diameter, and the balls are 2 to 20 mm in diameter, a filling of the cage with the balls being such that the balls do not form, at rest, superposed layers, the diameter of the balls being 2 to 10 times less than the diameter of the cage, preferably less than 2 to 8 times less than the diameter of the cage. - a reservoir of a foaming liquid, - a compressed gas cylinder , - a first pipe out of the reservoir and passing through a pneumatic pump while being connected to suction and discharge ports of said pneumatic pump, - a second pipe out of the cylinder and passing through an expansion valve , the first pipe and the second pipe being joined at a junction ; - a pipe for transporting a mixture of the foaming liquid and the compressed gas extending between the junction and a foam ejection end ; - a mixer of the foaming liquid and the compressed gas, in which the foam is formed, and located on the pipe for transporting the mixture between the junction and the ejection end ; the second pipe comprising a bifurcation leading to a port for starting the pneumatic pump, characterised in that the mixer comprises : - a cylindrical wall open at the two axial ends and through which the mixture passes between said ends axial, - a cage formed in the cylindrical wall, - and at least one solid body which is free to move in the cage, the at least one solid body consisting of 1 to 40 balls, constructed of rigid materials, and - a cylindrical wall open at the two axial ends and through which the mixture passes between said ends axial, - a cage formed in the cylindrical wall, - and at least one solid body which is free to move in the cage, the at least one solid body consisting of 1 to 40 balls, constructed of rigid materials, and in that the cage is 10 to 400 mm in length, 10 to 100 mm in diameter, and the balls are 2 to 20 mm in diameter, a filling of the cage with the balls being such that the balls do not form, at rest, superposed layers, the diameter of the balls being 2 to 10 times less than the diameter of the cage, preferably less than 2 to 8 times less than the diameter of the cage. <EOS>