Blackstone Ultrasonics was founded in the mid 1950's in the hope that the addition of ultrasonic agitation to a conventional clothes washer would revolutionize laundering of fabrics. It didn't take long before it was recognized that ultrasonic techniques weren't going to be effective - at least not by simply adding ultrasonic transducers to a conventional washing machine. Let's take a couple of minutes to understand the reason for the initial failures and what promising strides have been made in specialized areas since the 1950's trials. The problems with ultrasonic clothes washing are two-fold. First of all, fabrics are very good absorbers of ultrasonic energy. The use of draperies, carpets and upholstered furniture to prevent echos in homes and theatres attest to this fact. Because fabrics are so absorptive of sound, introducing enough ultrasonic energy to overcome the absorption would be a huge and expensive task. This fact alone makes the idea of ultrasonic laundering quite impractical. The second problem is that most fabrics are, themselves, flexible. In earlier blogs we discussed the fact that resilient materials like rubber and some plastics are difficult to clean ultrasonically for the same reason. Ultrasonics works best when it has a hard surface to work against. Thus the "double-whammy" against laundering with ultrasonics. Inevitably, the idea of an ultrasonic clothes washing machine has re-surfaced repeatedly over the years. In one exemplification, it was proposed that fabrics be presented to an ultrasonic cleaning device in a single layer to overcome the problem of ultrasonic absorption. Further, it was suggested that the fabrics be stretched (in some cases over a hard surface) to increase their ability to attract cavitation bubble implosions. In the case of stretching the fabric over a hard surface, the surface would actually act as the "receiver" of ultrasonic cavitation bubble implosions. The fabric would just be in the intense cavitation zone adjacent to the hard surface. After years of exploration, ultrasonic laundering is still not a practical application. There are very few garments (or other items made of fabrics) that can be presented in a single flat layer. A few exceptions, of course, are bed sheets, towels and other large, flat items which, unfortunately, represent a very small percentage of overall laundering requirements. The final nail in the coffin of the ultrasonic washing machine is, as is often the case, economics. At least at this point in time, even if there were an application that could be accomplished successfully and reasonably using an ultrasonic approach, the cost of the machine would likely be prohibitive. At least for the time being, it seems that sloshing or agitating clothes in a vat of soapy water has the market pretty much cornered. And finally, for those of you who have been wondering -- NO, I don't advocate hand washing clothes garment by garment with a washboard or rock. Dishes are one thing, clothes are another. For me, a clothes washing machine does a good job and makes sense as long as it is not over-challenged. But for that, there is stain stick!