Effective fluid front of the moving meniscus in capillary

Traditionally, the meniscus bottom is taken as the fluid front when tracking the fluid motion in capillary, but in simulation studies, the thus-calculated motion curve deviates notably from the modified Lucas-Washburn equation. Here, we report that, by considering a volume equivalent of the meniscus part, the motion of the equivalent front agrees very well with the theoretical prediction; furthermore, such an effective fluid front can be directly represented by a specific position of the meniscus, which is independent of the capillary radius. These findings provide an accurate and practical method for describing the motion of the fluid front in capillary.