The newer, Unicode and OpenType, toolchain gives you a lot of flexibility in what font you use for text and math, and lets you mix-and-match fonts and symbols in math mode. Here’s an example.
\documentclass[crop,varwidth]{standalone}
\usepackage{iftex}
\usepackage[ngerman]{babel}
\usepackage{mathtools}
\ifPDFTeX
\usepackage[T1]{fontenc}
\usepackage[scaled=0.92]{helvet}
\usepackage[helvet]{sfmath}
\renewcommand{\familydefault}{\sfdefault}
\else
\usepackage{unicode-math}
\setmainfont{Arial}
\defaultfontfeatures{Scale=MatchUppercase}
\setsansfont{Arial}
%% Change this line to change the math symbols:
\setmathfont{Asana Math}
%% Set various (but not all) Unicode tilde symbols:
\setmathfont[range={"007E,"02DC,"02F7,"0303,"0330,"0334,"033F,"034A,"0360}
]{TeX Gyre DejaVu Math}
%% Set the math alphabets:
\setmathfont[range=up]{Arial}
\setmathfont[range=it]{Arial Italic}
\setmathfont[range=bfup]{Arial Bold}
\setmathfont[range=bfit]{Arial Bold Italic}
\fi
\DeclareRobustCommand\d[1]{\mathop{d{#1}}}
\begin{document}
In text mode, \~{l} = \~{a}.
\( \begin{lgathered}
abc+\sum_{k=1}^{n}\int_{0}^{k}\sqrt{2}f(x)\d{x}\\
\tilde{l}= \sqrt{\widetilde{[a]}}
\end{lgathered} \)
\end{document}
Run this in pdflatex, and you will get the following, legacy-mode output, similar to David Carlisle’s:

Run it in xelatex, and it will set the main font to Arial (in case your thesis committee is checking the tail of the lowercase a), math letters to Arial, math-mode tilde symbols to TeX Gyre DejaVu Math, and other math symbols to Asana Math. Season to taste.

(You would need to change the first line to \documentclass[crop,varwidth,multi]{standalone} to get it to work in lualatex, but that would break the cropping.)
GUST should be coming out with a sans-serif math version of DejaVu Sans sometime soon, but for now, if you want a sans-serif math package, there are arev and arevmath.
$\tilde{a}$should work – egreg Aug 27 '13 at 16:33