The Ohnesorge number (Oh) is a dimensionless number that relates the viscous forces to inertial and surface tension forces. The number was defined by Wolfgang von Ohnesorge in his 1936 doctoral thesis.^[1][2]

It is defined as:

${\displaystyle \mathrm {Oh} ={\frac {\mu }{\sqrt {\rho \,\sigma \,L}}}={\frac {\sqrt {\mathrm {We} }}{\mathrm {Re} }}\sim {\frac {\mbox{viscous forces}}{\sqrt {{\mbox{inertia}}\cdot {\mbox{surface tension}}}}}}$

Where

• μ is the dynamic viscosity of the liquid
• ρ is the density of the liquid
• σ is the surface tension
• L is the characteristic length scale (typically drop diameter)
• Re is the Reynolds number
• We is the Weber number

The Ohnesorge number for a 3 mm diameter rain drop is typically ~0.002. Larger Ohnesorge numbers indicate a greater influence of the viscosity.

This is often used to relate to free surface fluid dynamics such as dispersion of liquids in gases and in spray technology.^[3][4]

In inkjet printing, liquids whose Ohnesorge number are in the range 0.1 < Oh < 1.0 are jettable (1<Z<10 where Z is the reciprocal of the Ohnesorge number).^[1][5]

• Laplace number - There is an inverse relationship, ${\displaystyle \mathrm {Oh} =1/{\sqrt {\mathrm {La} }}}$, between the Laplace number and the Ohnesorge number. It is more historically correct to use the Ohnesorge number, but often mathematically neater to use the Laplace number.