10. If the complex numbers $z_{1}, z_{2}, z_{3}$ satisfy $\frac{z_{3}-z_{1}}{z_{2}-z_{1}}=a \mathrm{i}(a \in \mathbf{R}, a \neq 0)$, then the angle between the vectors $\overrightarrow{Z_{1} Z_{2}}$ and $\overrightarrow{Z_{1} Z_{3}}$ is . $\qquad$
10. $\frac{\pi}{2}$