  With the aim of investigating the stability conditions of biaxial nematic
liquid crystals, we study the effect of adding a non-adsorbing ideal depletant
on the phase behavior of colloidal hard boardlike particles. We take into
account the presence of the depletant by introducing an effective depletion
attraction between a pair of boardlike particles. At fixed depletant fugacity,
the stable liquid crystal phase is determined through a mean-field theory with
restricted orientations. Interestingly, we predict that for slightly elongated
boardlike particles a critical depletant density exists, where the system
undergoes a direct transition from an isotropic liquid to a biaxial nematic
phase. As a consequence, by tuning the depletant density, an easy experimental
control parameter, one can stabilize states of high biaxial nematic order even
when these states are unstable for pure systems of boardlike particles.
