Hand washing removes pathogens mainly by mechanical action: friction and running water physically dislodge microbes and dirt from skin folds and lift them into the rinse water. Soap is not primarily a disinfectant here — it is a surfactant.

Surfactants & the lipid envelope

Soap molecules have a water-loving head and a fat-loving tail. They wedge into and dissolve oily grime, and — crucially for viruses like SARS-CoV-2 — the fatty tails can insert into and pry apart the lipid membrane that encloses many viruses, physically destroying the particle rather than "killing" it in the way a chemical disinfectant kills bacteria.

Why foam helps

Foam is a whipped-up dispersion of air bubbles whose walls are thin surfactant-stabilised films. Compared with a puddle of liquid soap, foam spreads over a far larger fraction of the skin's surface, reaches into creases and nail folds, and its bubble films trap loosened particles so they lift away with the rinse instead of sliding back onto clean skin.

Five zones, twenty seconds

WHO/CDC guidance recommends scrubbing for about 20 seconds, deliberately covering palms, backs of hands, between the fingers, thumbs and fingertips — the areas most often missed in a hurried wash, and disproportionately involved in touch-based transmission.

Reading this simulation

• Pick a zone, keep rub intensity and soap up, and the germ specks in that zone shrink and wash away as its progress bar fills.
• Try soap at 0% — mechanical rubbing alone still removes some germs, but removal plateaus well short of 100%.
• Guided auto-cycle steps through all five zones automatically, ~4 seconds each, matching the 20-second target.