For millennia, people washed clothes by stirring, hitting, squeezing and generally agitating them in rivers or buckets of water. The basic mechanism is to loosen dirt particles and use the water to wash them away or dissolve them.
Mostly, washing machines just automate the same process, agitating clothes in water, sometimes with detergent, to remove dirt from the fabric. Most use detergent to help free the dirt particles but more recently, some use ultrasound to create micro-cavitation bubbles and when they collapse, the shock waves help release the particles. That means the machines can clean at lower temperatures with little or no detergent.
It occurred to me that we don’t really need the machine to tumble the clothes. A ball about the size of a grapefruit could contain batteries and a set of ultrasonic transducers and could be simply chucked in a bucket with the clothes. It could create the bubbles and clean the clothes. Some basic engineering has to be done to make it work but it is entirely feasible.
One of the problems is that ultrasound doesn’t penetrate very far. To solve that, two mechanisms can be used in parallel. One is to let the ball roam around the clothes, and that could be done by changing its density by means of a swim bladder and using gravity to move it up and down, or maybe by adding a few simple paddles or cilia so it can move like a bacterium or by changing its shape so that as it moves up and down, it also moves sideways. The second mechanism is to use phased array ultrasonic transducers so that the beams can be steered and interfere constructively, thereby focusing energy and micro-cavitation generation around the bucket in a chosen pattern.
Making such a ball could be much cheaper than a full sized washing machine, making it ideal for developing countries. Transducers are cheap, and the software to drive them and steer the beams is easy enough and replicable free of charge once developed.
It would contain a rechargeable battery that could use a simple solar panel charging unit (which obviously could be used to generate power for other purposes too).
Such a device could bring cheap washing machine capability to millions of people who can’t afford a full sized washing machine or who are not connected to electricity supplies. It would save time, water and a great deal of drudgery at low expense.