Hidrodinamski pojavi pri izhlapevanju kapljic

Učinek kavnega obroča se pojavi v kapljicah, katerih kontaktna linija je zapeta na podlago. Izhlapevanje bi prosto kontaktno črto pomaknilo proti središču, ker pa podlaga tega ne dopušča, se pojavi radialni tok navzven, ki kompenzira izhlapelo tekočino. Ta tok je laminaren in odnaša morebitne nehlapljive delce na rob. Nastane usedlina, skoncentrirana na robu izhlapele kapljice. Marangonijev tok pa poganja gradient površinske napetosti, ki nastane zaradi temperaturnih razlik v tekočini ali pa zaradi razlik v sestavi ob površini večkomponentne kapljice. Ta tok je radialen in sklenjen.

Hydrodynamics in evaporating droplets

Coffee stain effect occurs in droplets with pinned contact line. Without pinning, the evaporation of fluid would move the contact line towards the centre. In the case with a pinned contact line the fluid near edge must be compensated with an outward radial flow. This is a laminar flow that carries potential colloid particles towards the contact line, resulting in an nonuniform stain after the fluid is evaporated. Marangoni flow is guided by the surface tension gradient, which occurs due to temperature gradient or concentration gradient in multicomponent droplets. The flow is radial and vortical.