Oil and water

\documentclass[tikz]{standalone}
\usepackage[francais]{babel}
\usepackage[T1]{fontenc}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[>=stealth]

\fill[orange!50](-2.5,0)rectangle (2.5,7);
\fill[blue!50](-2.5,0)rectangle (2.5,2.1);
\draw[very thick](-2.5,0)--node[left]{pas de flux}(-2.5,7);
\draw[very thick](2.5,0)--node[right]{pas de flux}(2.5,7);
\fill[blue!50]  plot[smooth, tension=.7] coordinates {(-2.5,2) (-1.5,2.5) (-0.5,2.5) (0.5,2.3) (2.5,2)};
\node at (0,4)[text width=3cm,text badly ragged,] { $p_w=2\times 10^5\mathrm{PA}$\\ $p_o=1$ };
\draw[->](0,-2)--(0,1);
\draw[->](0,5.5)--(0,9);
\node at (-2,6){huile};
\node at (-2,1){l'eau};
\node at (-2,-1){$S_o=0$};
\node at (2,-1){$p_w=2\times 10^5\mathrm{PA}$};
\node at (-2,8){$q_o=3\times 10^{-2}\mathrm{kg}/\mathrm{m^2s}$};
\node at (2,8){$q_w=0\times 10^{-2}\mathrm{kg}/\mathrm{m^2s}$};
\draw[<-](-5,5)--node[left]{{\footnotesize $y$}}(-5,4);
\draw[->](-5,4)--node[below]{{\footnotesize $x$}}(-4,4);

\end{tikzpicture}
\end{document}