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Sangwoo Shin

Think you know how clothes are cleaned in a washing machine? A common understanding is that soil particles are detached from dirty clothes primarily through the use of laundry detergent in a swirling flow. However, a recently published research paper by a University of Hawaiʻi at Mānoa engineering scholar reveals that the true “hero” in the dynamic cleaning process may actually be a sudden surge of the freshwater rinse, which provides accelerated particle removal from the small pores of fabric.

“The role of fluid flow in fabric cleaning has been regarded as a longstanding mystery in laundry detergency, so our discovery is significant that the pores within a fabric are so tiny that any fluid motion is nearly absent inside the pores,” said Sangwoo Shin, an assistant professor in mechanical engineering at the College of Engineering. “We’ve learned that it’s really the ‘change’ in the detergent concentration over time, through introduction of the freshwater rinse, that allows soil particles to be removed more effectively.”

Shin’s finding, which is a collaborative effort with researchers at laundry detergent giant Unilever in the United Kingdom and Princeton University in New Jersey, is published in the current issue of Physical Review Applied.

A video posted by Shin shows how stained fabric churned in fresh water is cleaned faster than the same stained fabric washed in detergent water.

“This demonstration confirms that fluid flow or even use of detergent is not the main driver for removing soil particles, but it is the sudden introduction of the freshwater rinse that results in cleaner laundry,” said Shin. “One practical way to achieve such a condition is to make sure that the spent water, or detergent-saturated water, is fully discharged before introducing fresh water for rinsing. Squeezing out the spent water from the laundry before rinsing could possibly help further, and would even be more helpful if the fresh water is supplied as fast as possible.”

The researchers believe that the finding may shed light on other applications that require the removal of particles and droplets from deep pores through the use of chemical gradients, such as in petroleum production or skin pore cleansing.

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