Impact of the apparent contact angle on the bubble departure at boiling

The contribution of the microregion (vicinity of the triple gas–liquid–solid contact line) to the force balance of a vapor bubble is discussed. The force balance of a bubble attached to the heater includes the capillary adhesion force that involves the contact angle. From the theoretical point of view, the microscopic value of the contact angle should be used in this expression. However in the experimental literature on the bubble departure, the apparent contact angle is commonly used instead. This usually leads to a substantial increase of the bubble adhesion and provides a good agreement with experimental data. A theoretical justification for such a substitution is proposed. The difference between the effective capillary adhesion force (that involving the apparent contact angle) and its true value is provided by the hydrodynamic stress caused by the liquid flow towards the contact line which is also at the origin of the difference between the microscopic and apparent contact angles.