Reference Work Entry

Encyclopedia of Microfluidics and Nanofluidics

pp 1513-1513

Ohnesorge Number

Definition

To account for the liquid viscosity in droplet formation, the appropriate nondimensional number is the Ohnesorge number which represents the ratio of internal viscosity dissipation to the surface tension energy. The Ohnesorge number may be written in terms of the square root of the Weber number divided by the Reynolds number. The Ohnesorge number is sometimes also referred to as stability number, viscosity number, Laplace number, or Z number. It is independent on the velocity and therefore only adequate to describe droplet ejection in conjunction with the Weber number.

The lower the Ohnesorge number the weaker are the friction losses due to viscous forces. This means that most of the inserted energy converts into surface tension energy, i. e. a droplet can be formed. The higher the Ohnesorge number the more dominant is the internal viscous dissipation. This means that most of the inserted energy converts into internal viscous dissipation, i. e. that a droplet formation is crit ...

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