hmm i have a theory on how Aizen controls the Espada, and Arrancar as a whole, now. after looking at this chapter... notice Aizen said "I cant' hear you, Grimmjow Jyagajaq" he said Grimmjow's full name. now when Wonderweiss was created, Aizen asked for his name. Wonderweiss replied with his full name. perhaps Aizen uses some type of kidou spell, or restriction, to be able to significantly weaken or even kill an Espada/Arrancar if he speaks his/her full name. If you notice, Grimmjow collapsed onto his knees after Aizen spoke his name. Now, that might have to do with reitatsu, but it could also be the full name reason. so my guess is either Aizen controls the Arrancar through intimidation with reitatsu, or the restriction, along with his ideals of no fear.