\documentclass{article}
\usepackage{amsmath}
\title{Complete Soft Matter Physics: Diffusion Equation}
\begin{document}
\maketitle

\section{Diffusion Equation}

For concentration $c(\mathbf r,t)$, diffusion is modeled by

$$
\frac{\partial c}{\partial t}=D\nabla^2 c.
$$

The diffusion constant $D$ sets the spreading scale
$\langle r^2\rangle \sim 2dDt$ in $d$ dimensions.

\end{document}
