# Weber number The Weber number weighs a flow's inertia against [[Surface_tension]]: We = ρU²L / σ. When it is small, surface tension wins and a body of [[Water]] holds itself together as a drop; when it is large, inertia wins and the drop flattens, stretches, and shatters. It is the number that decides whether a liquid stream becomes a spray, which makes it the governing group for fuel injection, crop spraying, inkjet printing, and every coating process on the [[WT!Advanced_Manufacturing_Center_of_Excellence|manufacturing]] bridge. ## The breakup ladder Raise We on a falling drop and it passes through a sequence anyone can recognise: sphere, flattened disc, bag that inflates and bursts, then a shredded cloud of fragments. Each transition sits near a threshold value, so a designer picks a nozzle [[Velocity]] to land in the regime they want. Too low and the spray is coarse and wets rather than covers; too high and drift carries it away, a real cost in [[WT!Northern_Agricultural_Center_of_Excellence|agricultural]] application. [[Viscosity]] modifies the picture, which is why Weber is usually quoted alongside the [[Reynolds_number]] rather than alone. ## Why small things are different Because We scales with the square of speed and with size, small slow things live in a world surface tension runs. A water strider stands on a pond; a droplet on a leaf beads instead of spreading; a bubble in a [[Porous_medium]] can plug a pore against a pressure gradient. This is the same lesson the [[Reynolds_number]] teaches about [[Viscosity]] at small scale, applied to a different [[Force]]. In the body it governs how [[Medicine|clinical]] aerosols deposit; in [[WT!Space_Mining_In_Minnesota|low-gravity handling]] it explains why liquids cling and will not pour. **Microsim brief (next pass):** One drop in a cross-flow; sliders for speed, size, tension. We readout, and the drop walks the sphere→disc→bag→shatter ladder as you push it. **On the spine:** [[Surface_tension]] · [[Dimensionless_quantity]] · [[Reynolds_number]] · [[Fluid_dynamics]] · [[WT!Thury_Hydrodynamics_Compendium]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Weber_number) : [Wikitube](https://en.wikitube.io/wiki/Weber_number) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_WT!Thury_Hydrodynamics_Compendium]], [[PORTAL_Physics]]. --- *Fluid seed wave · 2026-09-10 · seed register · microsim layer pending.*