  The size-dependent contact angle and the drying and wetting morphological
transition are studied with respect to the volume change for a spherical
cap-shaped droplet placed on a spherical substrate. The line-tension effect is
included using the rigorous formula for the Helmholtz free energy in the
droplet capillary model. A morphological drying transition from a cap-shaped to
a spherical droplet occurs when the substrate is hydrophobic and the droplet
volume is small, similar to the transition predicted on a flat substrate. In
addition, a morphological wetting transition from a cap-shaped to a wrapped
spherical droplet occurs for a hydrophilic substrate and a large droplet
volume. The contact angle depends on the droplet size: it decreases as the
droplet volume increases when the line tension is positive, whereas it
increases when the line tension is negative. The spherical droplets and wrapped
droplets are stable when the line tension is positive and large.
