  The optical and near-infrared (OIR) polarization of starlight is typically
understood to arise from the dichroic extinction of that light by dust grains
whose axes are aligned with respect to a local magnetic-field. The size
distribution of the aligned-grain population can be constrained by measurements
of the wavelength dependence of the polarization. The leading physical model
for producing the alignment is radiative alignment-torques (RAT), which
predicts that the most efficiently aligned grains are those with sizes larger
than the wavelengths of light composing the local radiation field. Therefore,
for a given grain-size distribution, the wavelength at which the polarization
reaches a maximum ($\lambda_\mathrm{max}$) should correlate with the
characteristic reddening along the line of sight between the dust grains and
the illumination source. A correlation between $\lambda_\mathrm{max}$ and
reddening has been previously established for extinctions up to $A_V\approx4$
mag. We extend the study of this relationship to a larger sample of stars in
the Taurus cloud complex, including extinctions $A_V>10$ mag. We confirm the
earlier results for $A_V<4$ mag, but find that the $\lambda_\mathrm{max}$ vs.
$A_V$ relationship bifurcates above $A_V\approx4$ mag, with part of the sample
continuing the previously observed relationship and the remaining part
exhibiting a significantly steeper rise. We propose that the data exhibiting
the steep rise represent lines-of-sight towards high density "clumps," where
grain coagulation has taken place. We present RAT-based modeling supporting
these hypotheses. These results indicate that multi-band OIR polarimetry is a
powerful tool for tracing grain growth in molecular clouds, independent of
uncertainties in the dust temperature and emissivity.
