  We study the unzipping of a double stranded DNA (dsDNA) by applying an
external force on a single strand while leaving the other strand free. We find
that the dsDNA can be unzipped to two single strands if the external force
exceeds a critical value. We obtain the phase diagram which is found to be
different from the phase diagram of unzipping by pulling both the strands in
opposite directions. In the presence of an attractive surface near DNA, the
phase diagram gets modified drastically and shows richer surprises including a
critical end point and a triple point.
