  Merging of isolated Bose-Einstein condensates (BECs) is an important topic
due to its relevance to matter-wave interferometry and the Kibble-Zurek
mechanism. Many past research focused on merging of BECs with uniform initial
phases. In our recent numerical study (Phys. Rev. A 97, 013612 (2018)), we
revealed that upon merging of rotating BECs with non-uniform initial phases,
spiral-shaped dark solitons can emerge. These solitons facilitate angular
momentum transfer and allow the merged condensate to rotate even in the absence
of quantized vortices. More strikingly, the sharp endpoints of these spiral
solitons can induce rotational motion in the BECs like vortices but with
effectively a fraction of a quantized circulation. This paper reports our
systematic study on the merging dynamics of rotating BECs. We discuss how the
potential barrier that initially separates the BECs can affect the profile of
the spiral solitons. We also show that the number of spiral solitons created in
the BECs matches the relative winding number of the rotating BECs. The
underlying mechanism of the observed soliton dynamics is explained.
