Adiabatic expansions of an ideal gas is accompanied by

$\mathrm{\Delta E}=\mathrm{\Delta Q}-\mathrm{W}$

For adiabatic expansion, $\mathrm{\Delta Q}=0$

$\Rightarrow \mathrm{\Delta E}=-\mathrm{W}$

The negative sign shows decrease in Internal energy, which is equal to the work done on the system by the surroundings.