2019-03-01ReadersDigest_AUNZ

54 | March• 2019

COMING CLEAN

particleswereremovedonlybecause

they drifted out randomly.

Butthatwasafar-from-satisfying

explanation. Calculations of diffu-

sionratesshowedthatevenafter

beingloosenedbydetergent,dirt

particleswouldneedhourstomove

outoftheircosycrevicesandbe

washedaway.Thisclearlycontra-

dicted visible evidence achieved by

both machine- and hand-washing

methods.

Shin’s realisation that washer-peo-

ple across the ages were not defy-

ingthelawsofphysicscamewhen

hebeganstudyingthediffusionof

particlescausedbyelectricfields

(a process known as electrophore-

sis) and by chemical concentration

gradients (chemophoresis). When

combined, the result is known as

diffusiophoresis.

The trick, it seems, is to rinse with

fresh water after using detergent. If

thereisfreshwaterattheopenend

ofaporefilledwithsoapywater,the

detergent molecules are drawn out,

bringing the dirt with them. Even

thoughthereisnofluidflow,tiny

electric fields and osmotic pressure

Scientists have proved

thatcleaningdirtyclothesisphysicallypossible.It’ssome-

thingthathousekeepersaroundtheworldhaveknownfor

alongtime,butuntilnow,sciencewasstumpedastoexact-

lyhowwashingworked,saysSangwooShin,leadauthorof a

study published in the journalPhysical Review Applied.

“Thekeymechanismforlaundry
detergent,whichwehavebeenusing
for thousands of years, is completely
different to what we thought,” says
Shin,aresearcherattheUniversity
of Hawaii, US.
“Peopleinthedetergentindustry
did recognise decades ago that there
must be something else going on
with fabric cleaning, but it has never
been identified.”
The problem concerned dirt in tiny
crevicesinfabric–poresthatcanbe
asnarrowasonemicronwideand
aredead-endsthroughwhichwater
cannot flow.
When the fabric is dry, dirty sub-
stancesgetdrawnintotheporesby
capillarywicking–thecombination
of surface tension and the adhe-
sive force between the dirt and the
material.
Oftenoilyandthereforerepellent
to water, this dirt can only be dis-
lodged by detergents, which form a
layer around the particle, negating
its water-repelling properties. But
rinsingdoesnotflushoutthepores
becausetheyaretinyculs-de-sac.It
was thus assumed that the offending PHOTO AND ILLUSTRATIONS: GETTY IMAGES