Technical Note: Evaporation dynamics of sessile liquid drops in still air with constant contact radius

Evaporation of sessile drops with constant wetting radius is investigated. In contrast to constant contact angle the temporal evaluation of the droplet volume can not be formulated in a closed form. We provide two approximations for initial contact angles below 90 deg which allow predicting the evaporation dynamics in practice easily. The derived linear approximation is suitable for small initial contact angles with a maximum relative deviation of 1% for contact angles below 30 deg . Further, we provide a non-linear algebraic approximation with a maximum relative error 0.3% in the entire range of contact angle considered.