  Disks around brown dwarfs (BDs) are excellent laboratories to study the first
steps of planet formation in cold and low-mass disk conditions. The
radial-drift velocities of dust particles in BD disks are higher than in disks
around more massive stars. Therefore, BD disks are expected to be more depleted
in millimeter-sized grains compared to disks around T Tauri or Herbig Ae/Be
stars. However, recent millimeter observations of BD disks revealed low
millimeter spectral indices, indicating the presence of large grains in these
disks and challenging models of dust evolution. We present 3\,mm photometric
observations carried out with the IRAM/Plateau de Bure Interferometer (PdBI) of
three BD disks in the Taurus star forming region, which have been observed with
ALMA at 0.89\,mm. The disks were not resolved and only one was detected with
enough confidence ($\sim3.5\sigma$) with PdBI. Based on these observations, we
obtain the values and lower limits of the spectral index and find low values
($\alpha_{\rm{mm}}\lesssim 3.0$). We compare these observations in the context
of particle trapping by an embedded planet, a promising mechanism to explain
the observational signatures in more massive and warmer disks. We find,
however, that this model cannot reproduce the current millimeter observations
for BD disks, and multiple-strong pressure bumps globally distributed in the
disk remain as a favorable scenario to explain observations. Alternative
possibilities are that the gas masses in BD disk are very low
($\sim2\times10^{-3}\,M_{\rm{Jup}}$) such that the millimeter grains are
decoupled and do not drift, or fast growth of fluffy aggregates.
