\begin{figure*}[bh]\begin{minipage}{174mm} \centerline{\includegraphics[bb=0 0 50 50,width=8.0cm]{t01ba98af8a4c3ad71a.jpg}} \centerline{(a) in GMAW process}\end{minipage}\\\\\\\\\vfill\begin{minipage}{174mm} \centerline{\includegraphics[bb=0 0 50 50,width=8.0cm]{t0103845d19099386e3.jpg}} \centerline{(b) in VPPA-GMAW process}\end{minipage}\caption{Droplet transfer in one period}\label{fig:3} % Give a unique label\end{figure*}