How to draw irregular shapes inside each other?

Question

Let's assume we have this irregular shaped object:

---

Minimum Working Example (MWE):

\documentclass[margin=5mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.pathmorphing}

\begin{document}
  \begin{tikzpicture}
    \draw plot[domain=0:350, smooth cycle] (\x:2+rnd*0.9);
  \end{tikzpicture}
\end{document}

---

Screenshot of the result:

---

Description of the issue:

How can I draw the exactly same shape around the initial one, but with a bigger radius to create some "skin" like layer around the first one?

In fact I need two more shapes of those, each of them with a bigger radius than the previous one. Furthermore, the shapes should have some fill (either color or better: Dots).

Background:

I want to visualize a soil particle where a biofilm is growing on the surface of the particle. In the end it could look like part of the following image, but in my case where the biofilm is growing around the whole particle:

In best case, we could add some arrow on the right side to visualize the streaming gas... :-)

---

Update (2019/02/03):

This approach inspired by an answer of "Surrounding an arbitrary path?" based on the definition of mypath is throwing errors:

\documentclass[tikz,border=3cm]{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.pathmorphing}
\tikzset{%
  draw seam allowance/.style 2 args={
    preaction={line width=1mm,line join=round,double distance=#1*2,draw=#2},
  },
  seam allowances/.style={%
    preaction={clip},
    preaction={draw seam allowance/.list={#1}},
    draw,%fill=white,
  },
  seam allowances/.default={{{2cm}{blue}}},
  invclip/.style={
    clip,
    insert path={
      {[reset cm] (-16000pt,-16000pt)  -| (16000pt,16000pt) -| cycle}
    },
  },
}
\begin{document}
\begin{tikzpicture}
  \def\mypath{
    (\x:2+rnd*0.9) -- cycle
  }
  \begin{scope}
    \begin{pgfinterruptboundingbox}
      \path [invclip] \mypath;
    \end{pgfinterruptboundingbox}
    \draw [seam allowances={{4cm}{black},{2cm}{black}},line width=2mm]
    \mypath ;
  \end{scope}
\end{tikzpicture}
\end{document}

Answer 1

The problem with your code is that \mypath is not a proper path. This is a proposal to fix this (presumably based on this answer). We build up a list of random coordinates and then use them in the path.

\documentclass[tikz,border=3cm]{standalone}
\tikzset{%
  draw seam allowance/.style 2 args={
    preaction={line width=1mm,line join=round,double distance=#1*2,draw=#2},
  },
  seam allowances/.style={%
    preaction={clip},
    preaction={draw seam allowance/.list={#1}},
    draw,%fill=white,
  },
  seam allowances/.default={{{2cm}{blue}}},
  invclip/.style={
    clip,
    insert path={
      {[reset cm] (-16000pt,-16000pt)  -| (16000pt,16000pt) -| cycle}
    },
  },
}
\begin{document}
\begin{tikzpicture}
  \pgfmathsetseed{12}
  \foreach \X in {0,20,...,340} 
  {\ifnum\X=0
  \xdef\mypath{(\X:2+rnd*0.9)}
  \else
  \xdef\mypath{\mypath (\X:2+rnd*0.9)}
  \fi
  }
  \begin{scope}
    \begin{pgfinterruptboundingbox}
      \path [invclip] \mypath;
    \end{pgfinterruptboundingbox}
    \draw [seam allowances={{4cm}{black},{2cm}{black}},line width=2mm]
    plot[smooth cycle] coordinates {\mypath };
  \end{scope}
\end{tikzpicture}
\end{document}