\fill[lightgray] (-6,2) rectangle (-4,3);
\fill[lightgray] (-1,2) rectangle (1,3);
\fill[lightgray] (6,2) rectangle (4,3);

%\node[font=\tt] () at (-5,2.5)  {METx};
%\node[font=\tt] () at (0 ,2.5)  {METx};
%\node[font=\tt] () at (5 ,2.5)  {METx};

\path [-{Latex[round,scale=0.75]}]
  (-1,2.75) edge [out=180,in=90] (-3.0,0);
\path [-{Latex[round,scale=0.75]}]
  (-1,2.5) edge [out=180,in=90] (-2.5,0);
\path [-{Latex[round,scale=0.75]}]
  (-1,2.25) edge [out=180,in=90] (-2.0,0);

\path [-{Latex[round,scale=0.75]}]
  (1,2.75) edge [out=0,in=90] (3,0);
\path [-{Latex[round,scale=0.75]}]
  (1,2.5) edge [out=0,in=90] (2.5,0);
\path [-{Latex[round,scale=0.75]}]
  (1,2.25) edge [out=0,in=90] (2,0);

\node[inner sep=0pt] () at (-3.3,1.25) {$C_{\mathrm{frg}}$};
\node[inner sep=0pt] () at (3.3 ,1.25) {$C_{\mathrm{frg}}$};

\node[anchor=south,inner sep=0pt] () at (-2.5,3.2) {$C_{\mathrm{c}}$};
\node[anchor=south,inner sep=0pt] () at (2.5,3.2) {$C_{\mathrm{c}}$};

\path [{Latex[round,scale=0.75]}-]
  (-4,2.125)  edge [in=180,out=0] (-2.5,2.05);
\path [] (-2.5,2.05)  edge [in=180,out=0] (-1,2.125);

\path [{Latex[round,scale=0.75]}-]
  (-4,2.375) edge [] (-1,2.375);
\path [{Latex[round,scale=0.75]}-]
  (-4,2.625) edge [] (-1,2.625);

\path [{Latex[round,scale=0.75]}-]
  (-4,2.875)  edge [in=180,out=0] (-2.5,2.95);
\path [] (-2.5,2.95)  edge [in=180,out=0] (-1,2.875);

\path [{Latex[round,scale=0.75]}-]
  (4,2.125)  edge [in=0,out=180] (2.5,2.05);
\path [] (2.5,2.05)  edge [in=0,out=180] (1,2.125);

\path [{Latex[round,scale=0.75]}-]
  (4,2.375) edge [] (1,2.375);
\path [{Latex[round,scale=0.75]}-]
  (4,2.625) edge [] (1,2.625);

\path [{Latex[round,scale=0.75]}-]
  (4,2.875)  edge [in=0,out=180] (2.5,2.95);
\path [] (2.5,2.95)  edge [in=0,out=180] (1,2.875);

\path [-{Latex[round,scale=0.75]}]
  (-0.9,2) edge [out=270,in=90] (-1.05,0);
\path [-{Latex[round,scale=0.75]}]
  (-0.6,2) edge [] (-0.6,0);
\path [-{Latex[round,scale=0.75]}]
  (-0.3,2) edge [] (-0.3,0);
\path [-{Latex[round,scale=0.75]}]
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\path [-{Latex[round,scale=0.75]}]
  (0.3,2) edge [] (0.3,0);
\path [-{Latex[round,scale=0.75]}]
  (0.6,2) edge [] (0.6,0);
\path [-{Latex[round,scale=0.75]}]
  (0.9,2) edge [out=270,in=90] (1.05,0);

\node[inner sep=1pt,fill=white] () at (0,1) {$C_{\mathrm{area}}$};

\draw (-7,0) -- (7,0);
\node[anchor=north] () at (0,0) {Substrate};

\draw[densely dotted] (-4,3) -- (-4,4.5);
\draw[densely dotted] (-1,3) -- (-1,4.5);
\draw[densely dotted] (1,3) -- (1,4.5);

\draw[densely dotted] (4,3) -- (4,4.5);

\draw[stealth-stealth] (-4,4.3) -- (-1,4.3);
\node[anchor=south, inner sep=1pt] () at (-2.5,4.3) {$s$};
\draw[stealth-stealth] (-1,4.2) -- (1,4.2);
\node[anchor=south, inner sep=1pt] () at (0,4.2) {$w$};
\draw[stealth-stealth] (4,4.3) -- (1,4.3);
\node[anchor=south, inner sep=1pt] () at (2.5,4.3) {$s$};