Ideally, to conserve soap the washer would be started without any soap, to get out as much grease as possible with only water. Afterwards, with soap they'll be a problem that the bubbles would eventually try to fill the entire washer; to prevent this, at the top there should be a screen made of some hydrophobic material, probably with many sharp surfaces to pop all the bubbles. Ideally these razor sharp bubble popping surfaces would be inside a safe to handle outer screen. It would be best to recirculate the air from the top of the washer to make bubbles at the bottom, which would tend to conserve heat and not let soap bubbles escape.
 
 

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