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Any suggestions on how to draw something like this picture using TikZ?

Thanks!

This isn't a complete answer. But as a proof of concept, I tried adapting this answer from Stroke with variable thickness to make something like the arrows you want.

documentclass[tikz,margin=0.5cm]{standalone}

usetikzlibrary{decorations,arrows.meta}



makeatletter

pgfkeys{/pgf/decoration/.cd,

    start color/.store in =startcolor,

    end color/.store in   =endcolor

}

pgfdeclaredecoration{width and color change}{initial}{

    state{initial}[width=2pt, next state=line, persistent precomputation={%

        pgfmathdivide{50}{pgfdecoratedpathlength}%

        letincrement=pgfmathresult%

        defx{0}%

    }]{}

    state{line}[width=.5pt,   persistent postcomputation={%

        pgfmathadd@{x}{increment}%

        letx=pgfmathresult%

    }]{%

        pgfsetlinewidth{x/20*0.005pt+pgflinewidth}%

        pgfsetarrows{-}%

        pgfpathmoveto{pgfpointorigin}%

        pgfpathlineto{pgfqpoint{.75pt}{0pt}}%

        pgfsetstrokecolor{endcolor!x!startcolor}%

        pgfusepath{stroke}%

    }

    state{final}{%

        pgfsetlinewidth{pgflinewidth}%

        pgfpathmoveto{pgfpointorigin}%

        color{endcolor!x!startcolor}%

        pgfusepath{stroke}% 

    }

}

makeatother



begin{document}



begin{tikzpicture}



draw [line width=2pt, decoration={width and color change,start color=black, end color=black}, decorate,] plot [smooth, tension=1] coordinates { (0,0) (2,0.5) (5,0.7) (3,1.5) (1,1.5) } node [rotate=100] {tikz draw [-{Stealth[length=7mm, width=10mm]}](0,0);} ;



end{tikzpicture}



end{document} 

Here is a slight extension of Milo's nice answer. The main differences (improvements?) are:

1. I rewrote Alain Matthes' nice answer to allow for arbitrary functions determining the line width. Previously it was a linearly dependence, now it can be an arbitrary function such as a sine. All you need to do is to say declare function={varyinglw(x)=1+6*sin(1.8*x);}. Apart from that, I made the code a bit more general, increased its speed (I think) and got rid of makeatletter since there was nothing that cannot be achieved with commands not containing @s.


2. This variation also does not nest tizpictures. Rather, the arrows a re attached in the usual way, and can be even bent.

Here's how this works

• You need to find a function that determines how thick the line is at a given position of the path. This function has the name varyinglw. It's argument runs from 0 to 100. So if the function has a maximum at 50, the path will have reached its maximal width in the middle. An example for a function with this feature is declare function={varyinglw(x)=1+6*sin(1.8*x);} below. If you want to use different functions, use scopes. Unfortunately I do not now how one can reset functions that are declared with declare functions. Therefore you should keep your functions local.


• If you have a very long path, you may want to increase the number of segments since otherwise it won't look smooth any more. This can be done by saying e.g. /pgf/decoration/varying line width steps=180 as in the examples below.


• Other than that you can use pretty much any path.

documentclass[tikz,border=3.14mm]{standalone}

usetikzlibrary{decorations,arrows.meta,bending}

begin{document}



pgfkeys{/pgf/decoration/.cd,

         start color/.store in=startcolor,

         start color=black,

         end color/.store in=endcolor,

         end color=black,

         varying line width steps/.initial=100

}

pgfdeclaredecoration{width and color change}{initial}{

 state{initial}[width=0pt, next state=line, persistent precomputation={%

   pgfmathparse{pgfdecoratedpathlength/pgfkeysvalueof{/pgf/decoration/varying line width steps}}%

   letincrement=pgfmathresult%

   defx{0}%

 }]{}

 state{line}[width=increment pt,   persistent postcomputation={%

   pgfmathsetmacro{x}{x+increment}

   },next state=line]{%

   pgfmathparse{ifthenelse(x<pgfdecoratedpathlength-14pt,varyinglw(100*(x/pgfdecoratedpathlength)),

    varyinglw(100*((pgfdecoratedpathlength-14pt)/pgfdecoratedpathlength))*(pgfdecoratedpathlength-x)/14) )}

   pgfsetlinewidth{pgfmathresult pt}%

   pgfpathmoveto{pgfpointorigin}%

   pgfmathsetmacro{steplength}{1.4*increment}

   pgfpathlineto{pgfqpoint{steplength pt}{0pt}}%

   pgfmathsetmacro{y}{100*(x/pgfdecoratedpathlength)}

   pgfsetstrokecolor{endcolor!y!startcolor}%

   pgfusepath{stroke}%

 }

 state{final}{%

   pgfsetlinewidth{pgflinewidth}%

   pgfpathmoveto{pgfpointorigin}%

   pgfmathsetmacro{y}{100*(x/pgfdecoratedpathlength)}

   color{endcolor!y!startcolor}%

   pgfusepath{stroke}% 

 }

}





begin{tikzpicture}[varying arrow/.style={-{Stealth[length=5mm,width=3.2mm,bend]},

postaction={/utils/exec=pgfsetarrows{-},decorate,decoration={width and color change}}

}]

begin{scope}[declare function={varyinglw(x)=1+6*sin(1.8*x);}]

draw[varying arrow] (0,0) to[out=45,in=150] ++ (1.5,0)  to[out=-30,in=-135] ++ (3,0);

draw[varying arrow,/pgf/decoration/varying line width steps=180]

(0,-3) to[out=45,in=150] ++ (1.5,0)  to[out=-30,in=-135] ++ (3,0) 

 to[out=45,in=45] ++ (-2,1);

draw[varying arrow,/pgf/decoration/varying line width steps=180,

/pgf/decoration/start color=red,/pgf/decoration/end color=blue]

(0,-5) to[out=15,in=165] ++ (2,0)  to[out=-15,in=90] ++ (1,-1) 

 to[out=-90,in=-20] ++ (-1.5,0);

end{scope}



begin{scope}[declare function={varyinglw(x)=4-3*cos(7.2*x);},xshift=6cm]

draw[varying arrow] (0,0) to[out=45,in=150] ++ (1.5,0)  to[out=-30,in=-135] ++ (3,0);

draw[varying arrow,/pgf/decoration/varying line width steps=180]

(0,-3) to[out=45,in=150] ++ (1.5,0)  to[out=-30,in=-135] ++ (3,0) 

 to[out=45,in=45] ++ (-2,1);

draw[varying arrow,/pgf/decoration/varying line width steps=180,

/pgf/decoration/start color=yellow,/pgf/decoration/end color=red]

(0,-5) to[out=15,in=165] ++ (2,0)  to[out=-15,in=90] ++ (1,-1) 

 to[out=-90,in=-20] ++ (-1.5,0);

end{scope}



begin{scope}[declare function={varyinglw(x)=4-3*cos(5.6*x);},xshift=12cm]

draw[varying arrow] (0,0) to[out=45,in=150] ++ (1.5,0)  to[out=-30,in=-135] ++ (3,0);

draw[varying arrow,/pgf/decoration/varying line width steps=180]

(0,-3) to[out=45,in=150] ++ (1.5,0)  to[out=-30,in=-135] ++ (3,0) 

 to[out=45,in=45] ++ (-2,1);

draw[varying arrow,/pgf/decoration/varying line width steps=180,

/pgf/decoration/start color=yellow,/pgf/decoration/end color=blue]

(0,-5) to[out=15,in=165] ++ (2,0)  to[out=-15,in=90] ++ (1,-1) 

 to[out=-90,in=-20] ++ (-1.5,0);

end{scope}

end{tikzpicture}

end{document} 

ALTERNATIVE: Some additional possibilities arise with the calligraphy library. (If I understand correctly, currently you need to download the latest version from this repository.)

documentclass[tikz,border=3.14mm]{standalone}

usepackage{spath3}

usetikzlibrary{calligraphy} % need to download the repository from https://github.com/loopspace/spath3

% run "tex spath3.dtx" and then copy the spath3.sty file in the same directory

% as the tex file

usetikzlibrary{arrows.meta,bending,decorations.pathreplacing}

tikzset{calligraph/.style={postaction={decorate,decoration={show path construction,

    moveto code={},

    lineto code={},

    curveto code={

      calligraphy (tikzinputsegmentfirst) .. controls

        (tikzinputsegmentsupporta) and (tikzinputsegmentsupportb)

        ..(tikzinputsegmentlast);

    },

    closepath code={}}}}

}

begin{document}

begin{tikzpicture}[line width=1pt]

pen (-135:.125) --  (45:.125) ;

draw[calligraph,{Stealth[length=5mm,width=3.2mm,bend]}-] (0,0) to[out=45,in=150] ++ (1.5,0)  to[out=-30,in=-135] ++ (3,0);

end{tikzpicture}



begin{tikzpicture}[line width=1pt]

pen (-135:.125) --  (45:.125) ;

draw[calligraph,{Stealth[length=5mm,width=3.2mm,bend]}-] (0,0) to[out=45,in=150]

++ (1.5,0)  to[out=-30,in=45] ++ (1,-2);

end{tikzpicture}

end{document}