Lotus Effect Simulator | mysimulator.uk

Biomimetic Surface Lab

Surface roughness 0.55

Hydrophobicity θᵧ 118°

Leaf tilt 18°

Contact angle θ_CB

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Droplets rolled off

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Surface cleanliness

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How it works

Lotus leaves stay clean because their surface is both waxy (chemically water-repellent) and covered in microscopic bumps. A droplet resting on those bumps only touches the very tips — most of its underside sits on trapped air — so it beads into a near-sphere instead of spreading into a film. That composite solid/air contact is described by the Cassie–Baxter equation:

cos θ_CB = f_s · (cos θᵧ + 1) − 1
f_s = 1 − 0.85 · roughness

where θᵧ is the flat material's own (Young) contact angle and f_s is the fraction of the droplet's base that actually touches solid wax rather than air. Rougher, waxier surfaces push f_s down and θ_CB up toward 180°, and a droplet with a higher contact angle needs far less tilt before gravity beats rolling friction and it rolls away — picking up loose dirt as it goes and leaving the leaf self-cleaned.

Surface roughness — height/density of the micro-bumps; higher roughness traps more air under a droplet, raising the contact angle.
Hydrophobicity θᵧ — how water-repellent the flat wax itself is, before texture is added.
Leaf tilt — the slope droplets have to overcome; too shallow and even a well-beaded droplet stays pinned in place.
Sprinkle dirt — scatters dust across the leaf so you can watch rolling droplets sweep it away (or, at low contact angle, spread it into a smear instead).

Real-world relevance: this is the working principle behind self-cleaning "lotus-effect" paints, water-repellent fabrics and anti-fouling solar-panel coatings — all engineered directly from the leaf's own micro-texture.

Water droplet Dirt / dust Waxy micro-bumps

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