Capillary number, also known as Ca, is a dimensionless number that describes the importance of viscous forces relative to surface tension forces in a fluid flow. It is calculated as the ratio of viscous forces to surface tension forces, and is commonly used in fluid dynamics and microfluidics to characterize the behavior of fluids in small channels or capillaries. A low capillary number indicates that surface tension dominates the flow behavior, while a high capillary number indicates that viscous forces are more significant. Understanding the capillary number is important in various applications, such as in the design of microfluidic devices, oil recovery processes, and modeling of droplet formation and breakup.