  Phase field theory is widely used to model multi-phase flows. A drop can
shrink or grow spontaneously due to the redistribution of interface and bulk
energies to minimize the system energy. In this paper, the spontaneous
behaviour of a drop on a flat surface is investigated. It is found that there
exists a critical radius dependent on the contact angle, the domain size and
the interface width, below which the droplet will eventually disappear. In
particular, the critical radius can be very large when the contact angle is
hydrophilic. The theoretical prediction of the critical radius is verified
numerically by simulating a drop on a surface with various contact angles, the
domain sizes and the interface widths.
