\begin{figure*}[bh]
\begin{minipage}{174mm}
\centerline{\includegraphics[bb=0 0 50 50,width=8.0cm]{t0103845d19099386e3.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*}