Adding a \vfill
as part of the \footnoterule
redefinition will push the footnote to the bottom of the page.
\documentclass{article}
\usepackage{lipsum}
\let\svfootnoterule\footnoterule
\renewcommand\footnoterule{\vfill\svfootnoterule}
\begin{document}
\lipsum[1-3]
This is a test\footnote{My footnote}
\lipsum[4]
\clearpage
\lipsum[1-3]
This is a test\footnote{My footnote}
\lipsum[3-4]
\end{document}
To clarify the approach, in response to a comment, the \let\svfootnoterule\footnoterule
stores a copy the original definition of \footnoterule
(which is the horizontal line above the footnote) under a new name for later use. Then, \renewcommand\footnoterule{\vfill\svfootnoterule}
redefines the \footnoterule
by adding stretchable glue (\vfill
) and then reinvoking the saved definition.