Potential of ultrasonics for energy saving in the household washing process

In the past few years, small household devices have appeared on the market for ultrasonic cleaning of textiles. In this study, the focus is on devices that can be immersed in a water tub for cleaning textiles by ultrasonics. These new devices were tested to see if they presented an opportunity to save electrical energy. Cleanliness levels were judged by comparison to conventional programs in household washing machines. The two devices tested demonstrated a higher effect than soaking without ultrasonics but a lower effect than any wash program. To get a better overview of what ultrasonics were capable of, additional experiments were conducted, i.e., in an ultrasonic bath for cleaning metal parts. It had a higher ultrasonic intensity and showed that there was still potential to get a higher cleaning effect for textiles at a low energy consumption level. It is concluded that, as far as household washing is concerned, ultrasonic treatments provide opportunities for the reduction of energy demand in low-temperature washing programs with a small textile load.


Introduction
Efforts are being made worldwide to reduce the consumption of fossil fuels. One way is to provide "green" energy, and another is to optimize the existing processes that would reduce energy consumption. Overall, the use of washing machines accounted for approximately 6.4% of household EU electricity consumption in 2013 (International Association for Soaps 2013), an amount of about 24.2 TWh (Pakula & Stamminger, 2010). Thus, optimization of the domestic washing process can lead to great energy savings.
From the perspective of innovation tools such as TRIZ (Chen and Liu 2001), ultrasonics seem to be a suitable alternative method to reduce energy consumption. However, most attempts to introduce ultrasonics in household washing machines to enhance or substitute the mechanics of the drum rotation have been discontinued in the past. Thus, a brief overview of the use of ultrasound in household washing machines is presented, and new devices using ultrasound have been investigated for their potential to reduce the use of electrical energy in the washing process.

Household washing process
The main parameters that affect cleaning performance in household washing machines are detergent action, mechanics of drum rotation, time, and temperature (Sinner, 1960). Depending on the combinations of the parameters, different programs are designed to suit different kinds of fabric.
Several factors are responsible for the energy consumption in the washing process. While the heating of the suds constitutes the lion's share of energy consumption, drum rotation and the short spin-drying process have a lower impact. Heating of suds can be accomplished by using hot water from the mains or by heating cold tap water in the machine. When the water supplied to the machine is cooler than the machine and its surroundings, some heat energy is also obtained through radiation and heat conduction from the machine's parts. However, this contribution is usually low as the time between the water inlet and the beginning of heating is short-between 5 and 10 min-and usually not calculated and reported in studies for washing machines. In this study and the majority of studies, the machine's heating was achieved by electricity. Water pumps, valves, displays, sensors, and controls play a minor role in energy consumption. As all these devices are also electrically controlled, in this report energy consumption is identical to the measured electrical energy consumption.
The cleaning of durable cloth, such as cotton textiles, is usually done in horizontal axis washing machines, which are the European market standard. The main contribution of the machine to the cleaning effect is the rotation of the drum during the main wash phase for about 30 min to 3 h. A higher temperature allows achieving a certain cleaning performance in a shorter time or a higher cleaning performance. The detergent helps to achieve a high level of cleaning.
For the cleaning of delicate items, in particular wool, these machines have special programs that mimic washing by hand and reduce the movement of the textiles to a minimum. Wool becomes felted and shrinks when its fibers rub against each other in the suds. This can be prevented through a chemical modification of the fibers. However, as wool is traditional cloth and one of the ongoing fashion trends is to use unaltered natural fibers, wool shrinkage is still a problem.
Wool programs are more or less soaking processes with nearly no drum movement. Some do only a partial rotation, and some do only one complete rotation at intervals of several minutes.
Cotton programs have the highest drum movement, at about 50 rotations per minute. At the end of both programs, a short period with a much higher rotation speed takes place to spin-dry textiles. Centrifugal forces are so high in this period that the clothes are pressed onto the walls and no friction occurs between textiles. Thus, no fiber damage is witnessed in this last stage of the process, but no additional cleaning effect takes place either.
Cotton programs often have a lower water-totextile ratio than wool programs. The first reason is that washing machines, by design, have a dead volume under the drum that must be filled before the textiles can be sufficiently saturated with the suds. Because of this, programs such as the wool program, for which a low load is recommended, have a higher water-to-textile ratio than programs with a high load such as the cotton program. The second reason is that a high water level is generally preferred in wool programs because the buoyancy of the textiles in the suds and the greater distance between them reduces friction. On the other hand, the energy consumption of wool programs is not subject to EU regulations and is rarely examined in consumer tests. Therefore, increased energy consumption due to a high water-totextile ratio is hardly noticeable.
Heating of suds to higher temperatures dominates the consumption of energy. For example, in the Bosch WCM69 washing machine, measurements showed that the energy consumption for a wool program changes from 0.01 kWh at 20 °C to 0.23 kWh at 40 °C because suds have to be heated.
The mean temperature of washing programs in Europe is about 41 °C (International Association for Soaps 2013). The average washing temperature, however, has decreased over the past few decades for the following reasons: • EU regulations regarding energy consumption, rating of appliances, and mandatory availability of low-temperature programs (Boyano et al., 2020) • Increasing share of machines with low-rated energy consumption (Bertoli, 2017;Yilmaz et al., 2019) • Marketing of detergent and manufacturing of machines with claims of low-temperature benefits (Mylan, 2017) • Consumer concerns over environment and rising energy prices On the other hand, consumers also expect a high cleaning performance, so they do not use only cold or 20 °C programs, especially when it comes to washing woolens with a low mechanical cleaning action. According to a survey in Finland, more than 75% of consumers use temperatures of 30° or more for wool washing (Laitala & Klepp, 2016).

Ultrasonic cleaning
Ultrasonics are used in many applications to clean hard surfaces immersed in a liquid solution. Above a threshold of about 0.5-6 W/cm 2 , ultrasound cavitation occurs, which is the main mode of cleaning (Ensminger & Bond, 2011). When steam bubbles formed in this process collapse near a surface, adhering soil particles are removed by local fluid currents. As the local effect is very strong, some degree of surface degradation is usually observed. Nevertheless, this process is a standard application for cleaning metal parts, for example.
For soft surfaces, the application is more difficult because cavitation occurs predominantly at phase boundaries with a high difference in density. For metal-water boundaries, cavitation is a common effect, whereas the textile-water surface does not show this effect. In this case, cavitation has to be enabled throughout the liquid bath, requiring a highpressure field. Another effect is that textiles trap some amount of air, which dampens the ultrasonic field (Moholkar & Warmoeskerken, 2004).

Ultrasonic action in textile cleaning
Not many household washing machines claim to use ultrasonics, and those that do are mostly top loaders with a vertical drum axis. Vertical axis machines use more water than European horizontal axis machines; therefore, it is easier to apply ultrasound because textiles are immersed in water. Around the year 2000, Sanyo introduced a washing machine, ASW-HB 700D, which claimed to use ultrasonics and a nodetergent washing cycle. It was compared in a study to a standard washing machine; but the study lacked details and comparability as a vertical axis machine with ultrasonics was compared to a horizontal axis machine without. Also, the study failed to characterize the ultrasonics that were used during the operation (Wüstemann et al., 2006). The system was discontinued in the following years.
The washing machine, WVMD1208AHG, from Whirlpool is said to speed up detergent dissolution by ultrasonics. The one examined in our laboratory had no ultrasonic transducer but only a membrane pump to introduce air into the suds. No ultrasonics could be measured during the program. X10 from TCL had only an additional ultrasonic bath at the top of it, and it was not intended to clean textiles but to clean items like glasses or jewelry. A realistic use of ultrasonics for laundry was only found in WT EON 650 from Godrej (Godrej appliances announces launch of new washing machines, 2013). It had an ultrasonic transducer at the top of the appliance, which could be used to do a manual pretreatment, but it did not claim to use ultrasonics in the main wash process in the drum.
There are many laboratory experiments about cleaning textiles with ultrasonics. They were usually conducted in devices used for other purposes, such as ultrasonic cleaning baths for cleaning metal parts (Wang et al., 2021). However, most of these studies lack suitable references to typical washing programs, a reasonable selection of stains, and the ability to measure ultrasonics.
The effects of different frequencies of 40, 60, 80, and 100 kHz were examined in Wang et al. (2021). However, neither the ultrasonic efficiency of the transducer nor the temperature was reported. Only beverage stains were used to test the cleaning efficiency. Thus, there are still some uncertainties about the best-suited frequency.
The damage caused to different fiber types by ultrasonics was examined in Wang et al. (2021) and Muhammet (2013). No change in tensile strength (Wang et al., (2021) or abrasion resistance (Muhammet, 2013) was observed, but there was an alteration in the thermophysical properties of the fibers (Muhammet, 2013). The effect of ultrasonics on delicate textiles such as wool (Gotoh et al., 2015) and silk (Ma et al., 2014) was studied and considered less damaging than traditional washing of wool by hand (Hurren et al., 2008) or machine washing of silk (Ma et al., 2014).
In Kimmel et al. (2022), devices for the pretreatment of stains were examined. If a pencil-shaped ultrasonic device is used for a manual 30 s pretreatment of 20 different consumer-relevant stains of 5 × 5 cm 2 each, followed by a 20 °C cotton program, the 33 Page 4 of 11 Vol:. (1234567890) average stain removal level achieved is the same as a 40 °C cotton program without pretreatment.
In wool programs, the energy-efficient cleaning effect due to the drum rotation cannot be used. This leads to very low cleaning performance compared to, for example, cotton programs. The reduced drum movement is also reflected in the energy consumption of wool programs at 20 °C that do not have to heat suds and in which the motor is the main reason for energy consumption.
As the cleaning effect of the wool program is limited due to the reduced friction, this is also the reason why consumers often choose higher temperature levels in wool programs. Thus, for this kind of program and other similar programs with a low mechanical impact, such as the ones for delicates, the use of ultrasonics could provide an opportunity for making eco-efficient improvements in overall cleaning performance.
In recent years, many small devices claiming to use ultrasonics for washing have appeared on the market, which are designed for cleaning small loads. In this study, a selection of these devices was tested. Two typical devices were selected that can be immersed in a tub as an alternative washing process outside the washing machine. Conventional washing processes, on the other hand, are usually not very energy efficient for small loads as they need a minimum amount of water and the program structure is not very flexible in regard to the load. One pencil-shaped device was additionally tested, which was not intended to be immersed. It was originally intended to be used for a pretreatment. However, in this study, the device was not used as a pretreatment but as a method to get rid of visible stains, for example, by only treating single annoying spots and omitting the need to do a complete conventional washing process.
The aim of the study was to test whether these new devices show a sufficient cleaning effect at a lower energy consumption level compared to conventional washing programs. The test conditions were selected so that the highest possible cleaning effect could be achieved without the interference from additional load due to effects like dampening of ultrasound.

Materials and methods
The devices available in the market, which can be submerged under water, can be divided by the shape of the device into ellipsoid and cylindrical. At the beginning of the project, a market survey was done, which yielded 8 hits for cylindrical products and 32 for ellipsoids. Devices of similar types often resemble each other very strongly, indicating that similar devices are sold under different brand names.
Two samples from the most common forms were chosen for the test. The first device was ellipsoidal and meant to resemble a soap bar called Dolfi (Fig. 1, number 1) from Dolfi Sonic International Ltd., and the second is the cylindrical PJ-XY-03 from Peng Ju (Fig. 1, number 2). The devices were pretested to see if they showed a cavitation effect on an aluminum foil in a 1 L water bath within 5 min of operation. Device ellipsoid cylindrical-shaped pencil-shaped 1 2 3 Fig. 1 Devices 1 and 2 are claimed to clean textiles in a tub and work immersed in water. Device 1 from Dolfi is shown as ellipsoid. Device 2 Peng Ju is cylindrical in shape. Device 3 is for a manual pretreatment of textile stains Energy Efficiency (2023) 16:33 Page 5 of 11 33 Vol.: (0123456789) 1 did not show a cavitation effect, whereas device 2 did. For comparison with other technologies that use ultrasonics, a manual treatment was performed with device 3 (Fig. 1, number 3) analogous to the procedure in Kimmel et al. (2022). The difference between the approach in this study and in Kimmel et al. (2022) was that in this study the cleaning performance was assessed directly after the treatment and a manual rinsing step with tap water. In (Kimmel et al., 2022), the cleaning performance was assessed after a pretreatment in combination with a consecutive washing cycle in a conventional washing machine.
Device 4 is a conventional ultrasonic bath, Bandelin Sonorex RK 510, which had an ultrasonic generator that worked at a nominal working frequency of 35 kHz (Fig. 2). The transducers were fixed below the bottom of the steel bath.
All ultrasonic devices were characterized by their working frequency (Table 1). Measurement was done at a scanning rate of 1 MHz with a cavispector from Köchel Verifications GmbH.
To be able to compare and judge the results, additional experiments in conventional washing machines without ultrasonics were carried out ( Table 2). The wool program at 20 °C was performed in appliance 1, the cotton programs at 20 °C and 40 °C in appliance 2. Cotton at 40 °C is one of the most frequently used programs (Electrolux Group, 2021); therefore, the cleanliness level of this program could be regarded as satisfying in most cases from the viewpoint of the consumer.
The liquid detergent Persil Universal Gel from Henkel AG & Co. KGaA was used, which contains surfactants, builders, enzymes, and some other minor components but no bleach.
To measure washing performance, two stains made on cotton textiles, either with sebum/pigment (10D) or olive oil/pigment (10B), were obtained from wfk-Testgewebe GmbH. The stain olive oil/pigment was chosen as it responded very well to ultrasonics, according to Kimmel et al. (2022). Sebum/pigment was used because sebum is a typical stain found in textiles that have contact with human skin. Stains were cut into pieces of either 5 × 5 cm 2 or 10 × 10 cm 2 .
The stains contained additional dark pigments to make it easier to measure stain removal by brightness. The cleaning effect was assessed by CIELAB lightness L * , according to DIN EN ISO 11664-4 (2019). L * was measured with the reflection spectrophotometer Datacolor 600 from Datacolor Inc. The value L * = 0 corresponded to black and 100 to white. Higher values of L * indicated a better cleaning effect as the darker, colored stains were removed from the white textile substrate.

Experiments
In the experiments with devices 1 to 4 that use ultrasound, the most favorable conditions for a high cleaning performance were chosen: a low load of soiled textiles, the absence of obstacles between the textile and ultrasonic sound emitter, and a high water-to-textile ratio. For this purpose, no ballast load like additional textiles without stains was added. In the washing machines, additional clean textiles were used. In the case of the cotton programs, clean textiles made of cotton and polyester were used as a ballast load.
The ultrasonic devices 1 and 2 were tested in a steel bath measuring 30 × 24 × 15 cm 3 and having 7 L with a solution of 6 g detergent per liter at a temperature of 23 ± 1 °C. The steel bath is part of device 4, which was turned off during the tests with devices 1 and 2. Device 1 or 2 was placed in the middle of the bottom. The stains were positioned vertically with the help of metal wires at a distance of approximately 2 or 5 cm from the devices (Fig. 2). In each run, four stains were used, two of each type of stains. In the experiments with devices 1, 2, and 4 and in the soaking process, stains with the dimensions 10 × 10 cm 2 were used, as it was easier to attach them to the wires.
For device 3, stains with the size 5 × 5 cm 2 were used similar to the procedure in Kimmel et al. (2022). During the treatment of stains with device 3, the textile is not submerged under water but wetted with the detergent solution. The whole surface of each stain of 5 × 5 cm 2 was treated manually with direct contact between the pencil-shaped device 3 and the textile for 30 s.
When device 4 was tested, the same configuration was used as for devices 1 and 2.
The experiments with devices 1-4 and for soaking were repeated five times.
Cotton programs at 20 °C and 40 °C were performed in a W1935WPS-WTL washing machine from Miele & Co. KG, the wool program, at 20 °C in a WCM69 from Robert Bosch Hausgeräte GmbH. Clean towel was used to hold the stained textiles with the size 5 × 5 cm 2 . Due to the use of a towel, the load in the wool program was approximately 0.2 kg.
The main reason to use a towel was not to simulate a higher load but to avoid a high variance of stain removal, as the small textiles tend to curl up or to be trapped at the door sealing. As the use of the cotton program was intended as a comparison for consumer-relevant washing processes, an additional ballast load of clean textiles was used to get a typical high consumer load of 5 kg, which corresponds to approximately 3/4 of the maximum load. In this case, the ballast load was used to get a medium washing performance, as the cleaning performance of the cotton program depends on the load. A load of 0.2 kg as used in the wool program would have resulted in a too small washing performance in the cotton program.
In the cotton programs, more stains were used, because these programs were intended as a comparison to more consumer-relevant conditions. The additional stains will only slightly affect the cleaning performance, but reflect better the conditions found in household laundry washing as even textiles without visible stains contain some soil. The total area of stained textiles used corresponds to 70% of the recommended addition of stains according to DIN EN 60456. Another reason for using more stains was that cotton programs, without additional soil, show a strong foam development which disturbs program flow and dampens the mechanical action of the drum rotation on textiles. The experiments were repeated Table 1 Devices with ultrasonic that were used to clean stained textiles. Devices 1 and 2 were intended to clean laundry in a tub, device 3 for a manual pretreatment of stains, and device 4 was used to compare the results of devices 1 and 2 to a setup with a higher ultrasonic intensity five times with four stains fixed on a towel, two from each stain type.

Results
The conditions in the different treatments of the stained textiles are compared in Table 3. The experiments with the ultrasonic devices were carried out at a room temperature of about 23 °C. During treatment 4, with the ultrasonic bath turned on, water had to be cooled to keep the temperature within the designated range. The temperature increased by approximately 0.4 °C/min due to the work of the ultrasonic transducers. The temperature was manually kept constant at 23 ± 1 °C by periodically replacing a part of the solution with fresh detergent solution, this way the detergent concentration was kept constant. As temperature enhanced the cleaning effect, the cleaning effect was underestimated by this experiment as the temperature increase was limited.

Cleaning effect
The order of effectiveness of cleaning for the stains sebum/pigment and olive oil/pigment is almost the same (Figs. 3 and 4).
The smallest cleaning effect was observed during soaking in treatment 5; in this process, neither mechanical forces nor ultrasound was used. This value can be regarded as a reference for the lowest possible cleaning effect when doing manual washing by using a detergent in an aqueous solution. For a household washing machine, the lowest possible washing performance is the wool program at 20 °C in treatment 7 because the process resembles soaking and the drum movement is minimal compared to all other washing programs.
The cleaning effect of the two ultrasonic devices 1 and 2 was on the same level, according to ANOVA and post hoc Tukey test with = 5% . They were at a higher level than that of soaking but at a lower level compared to the wool program at 20 °C.
To see if the effect was too small due to the limited use of energy by the ultrasonic devices 1 and 2, a test under the same conditions with device 4 was made, a typical ultrasonic bath. In this case, the cleaning level is higher than that of the 20 °C wool program but lower than a 20 °C cotton program. It is, however, closer to the 20 °C cotton program. Thus, the effect of ultrasonics in a water bath can be enhanced to a higher level if devices with a higher ultrasonic intensity are used.
The use of device 3 is a completely different approach, as it was originally intended for manual pretreatment. For the olive oil/pigment stain, it showed the same cleaning level as the cotton 20 °C Table 3 Comparison of different treatments in this study. In all treatments, the detergent concentration was 6 g/L US indicates if ultrasonic was used in this treatment ( +) or not ( −) T max maximum temperature that was held for at least 5 min * In the main wash process step ** With option short # Each of the 4 stains was treated for 30 s program, and for sebum/pigment, even a higher cleaning level than cotton 40 °C. As expected, the cotton 40 °C program shows the highest cleaning performance of the washing programs, but the step between cotton 40 °C and 20 °C is not as high as that between cotton 20 °C and wool 20 °C.

Energy consumption
For the soaking process, a value of 0 kWh was applied, as in this study, the temperature increase of the tap water to room temperature was not taken into account.
The energy consumption of device 3, originally intended for a pretreatment, is very small for the Fig. 3 Cleaning effect on olive oil/pigment stain of ultrasonic treatments (solid border line, not hatched) and standard washing programs or soaking as reference (dashed border line, not hatched). The treatment numbers according to Table 3 correspond to the device numbers 1 to 4; for soaking and the programs wool 20 °C and cotton 20° and 40 °C, the treatment numbers are 5 to 8 treatment of four stains with the size of 5 × 5 cm 2 each. As most of the washing loads in typical households have only a few visible stains and consumers tend to keep the time for pretreatment short, it is not expected that a manual treatment will contribute a big share to the energy consumption. Even if a bigger area corresponding to stains with the size of 10 × 10 cm 2 was treated, the energy required would still account for less than 0.5% compared to the energy consumption of a cotton 20 °C program.
Device 1 needed half the energy and device 2 more energy than that for the wool 20 °C program, but both showed a lower cleaning level than that of the wool program. Device 4, the ultrasonic bath, showed a much higher cleaning level than the wool program. Although the bath needed fourfold energy of the wool program, the cleaning effect was still less than 25% of a cotton 20 °C program. The relationship between energy consumption and cleaning effect for all treatments is summarized in Fig. 5.

Discussion
The ultrasonic devices showed a very broad spectrum of cleaning levels at a low energy consumption level, as they used neither drum rotation nor suds heating.
Devices 1 and 2 had a slightly better effect than soaking, but it seemed unlikely that they would meet consumer expectations, as the cleaning effect was even lower than that of a wool program at 20 °C.
The ultrasonic bath, which was used as a test to see whether intensification of ultrasonic could yield a higher cleaning effect, had a cleaning level between the wool program at 20 °C and the cotton program at 20 °C. As this was a test with a very low load, the results need to be verified in household appliances to determine under what circumstances and at what level of textile load this effect could be achieved.
A manual treatment of stains with device 3 only makes sense if visible stains can be perceived. Lightly soiled items are defined by the German industry association IKW as laundry without visible stains, and even medium soiled is defined as having only a few light stains (Industrieverband Körperpflege-und Waschmittel e. V, 2017). Therefore, treatment would be possible for medium or more heavily soiled textiles. As it can be expected that consumers tend to keep the time for treatment short, it is not expected that the treatment will contribute a big share to energy consumption. An area of application of device 3, as it was used in this study, would be to remove stains from single textiles with an additional manual rinse or a rinsing process in a washing machine. In this case, the use of a full washing program for a single textile would be avoided.
The recommendation in the manual of the manufacturer was to pretreat single stains in heavily soiled laundry that could not be removed without a pretreatment. Another application is to use the pretreatment to change a mixture of textiles that would be rated as  heavily soiled to medium soiled by the pretreatment. In this case, a more gentle washing process as a second step can be chosen, which will enable, in combination with the low energy consumption of the pretreatment, an overall lower energy consumption. Critically, it should be noted here that the washing load in processes with ultrasound in this study was not chosen to reflect typical loads in consumer households, but to see the maximum effect of the new devices 1 and 2 under very favorable conditions for cleaning performance. The stained textiles have only a negligible fraction of the weight of typical cloths in the household. One stained textile with the size of 10 × 10 cm 2 has a weight of about 2 g, whereas a typical T-shirt has a weight of more than 100 g. The amount of dirt may be comparable for slightly soiled T-shirt, but the weight and dimensions of the textile are not comparable.

Outlook
To reach a conclusion about the potential of ultrasonics for new washing programs, we will first look at programs for tough textiles and high loads, such as the cotton program. In (Kimmel et al., 2022), it was shown that it is possible to use ultrasonic pretreatment for a low temperature program. This enables the consumer to use a 20 °C program instead of a 40 °C program for a textile load that contains only a few visible, hard-to-remove stains. However, this depends solely on consumer behavior.
The use of ultrasonics directly in the drum during a cotton cycle with a high load of textiles still seems unfeasible (Gallego-Juarez et al., 2010). There are no such appliances in the market, and in the past, machines with this claim were discontinued. The main argument is that the mechanical drum rotation has a very high cleaning effect for low-to-high loads and does not need that much energy compared to suds heating. If a comparable cleaning effect has to be achieved only by using ultrasonic sound, the energy input will have to be at a very high level, which will have to be determined in separate studies.
Programs like cotton allow the biggest loads and require the least amount of liquid. Modern horizontal axis machines in Europe need only about 2-3 L of liquid per kg of textiles in the main wash process. So textiles are not immersed in a water bath like in the experiments in this study. Thus, ultrasonic propagation is hindered, and additionally, strong dampening occurs because of several layers of textiles and trapped air in the textiles and the suds. One method to reduce trapped air in the suds would be to lower drum rotation. However, as drum rotation in cotton programs is optimized to achieve a high cleaning performance, this would reduce the combined cleaning performance.
Device 3 showed that the direct contact of a transducer surface with textiles had a high cleaning performance. Up to now, there are no washing machines using this principle in the drum. Analogous to a manual treatment, an ultrasonic-sound-emitting surface could, for example, be fixed into the door as proposed by different patent applications (Frucco & Sburlino (1987)); Chindyasov (2013)). In this case, the textiles would accidentally come into contact with the surface when laundry is moved in the drum. In this case, the direct contact with stains on textiles would be comparable to device 3. However, only a small part of the textile surface of the whole load will accidentally come into contact with the ultrasonic-sound-emitting surface for a short time, and cleaning is expected to be uneven as the effect is usually limited to only a few layers of textiles and requires some time to be noticeable.
For delicate textiles like wool, which cannot be washed by using high mechanical forces from drum rotation and for which smaller batches than cotton are recommended, ultrasonics seem to be an alternative option of operation. These programs typically have a higher water-to-textiles ratio and a lower absolute textile load when compared to cotton programs. The appliances used in this study had a water-to-textile ratio of approximately 5-8 L per kg of textiles in the main wash phase.
In this study, we showed that the cleaning level of the tested devices 1 and 2 was, even at very favorable conditions, lower than that of a wool program at 20 °C. However, in experiments using an ultrasonic bath with a higher ultrasonic input, the cleaning level could be enhanced. For an application of ultrasonics in the washing process, either as a separate device or integrated in a washing machine, the next step would be to determine how much energy would have to be used to balance the dampening by a consumer-relevant load to get a sufficient cleaning performance.
Funding Open Access funding enabled and organized by Projekt DEAL.