How to change font family in footnote?

Footnotes are typeset using (surprise) \footnotesize and the current font family. With the standard document classes, you may change this to, say, \small\sffamily by partially redefining the \@footnotetext command (Note: I'm using the etoolbox package for convenience):

\documentclass{article}

\usepackage{etoolbox}
\makeatletter
\patchcmd{\@footnotetext}{\footnotesize}{\small\sffamily}{}{}
\makeatother

\usepackage{blindtext}

\begin{document}

\blindtext\footnote{\blindtext}

\end{document}

A small drawback of this solution is that the footnote label is not affected by these changes.

The KOMA-Script-classes allow for a more elegant solution without the drawback mentioned above:

\documentclass{scrartcl}

\addtokomafont{footnote}{\small\sffamily}

\usepackage{blindtext}

\begin{document}

\blindtext\footnote{\blindtext}

\end{document}

EDIT: As Vincenzo pointed out, using the footmisc package is also possible:

\usepackage{footmisc}
\renewcommand*{\footnotelayout}{\small\sffamily}

This works with standard and KOMA-Script-classes, but also has the drawback that it won't affect the footnote label.

According to your comment, I'll answer not the question you asked, but what I think is the "right question" in this case, namely: "how to I change the default font for a (portion of a) document?"

The answer is: change the definitions of the various \stuffdefault macros---for a complete list of the possible values of stuff, see section 41.2 in source2e.pdf (try texdoc source2e to access this document). As an example, in this case I guess you want to do

\renewcommand\familydefault{\sfdefault}
\renewcommand\sfdefault{phv}
\normalfont

Beware, the value for \familydefault is a command, while the value for all other macros are only letters (without backslash) that will be used to form the complete logical name of the font. (Actually, it could be only letters too, but the use of a command provides an additional level of indirection that is usually welcome.) The final \normalfont command ensures that the modifications take effect immediately.