  We use high-speed imaging to study the effect of thixotropic aging in drop
impact of yield-stress fluids on pre-coated substrates. Our results reveal that
drop splashing is suppressed for "aged" compared to "unaged" samples,
indicating that thixotropic breakdown timescales during impact are long enough
to affect the dynamics. We propose and test several hypotheses for modifying
the dimensionless group ${\rm IF}(D/t)$ [1,2] to account for thixotropic aging.
The main challenge is that the steady flow properties (Herschel-Bulkley model
parameters) used in the current dimensionless group cannot be defined or
measured for thixotropically aged samples, because any deformation inherently
rejuvenates and breaks down the microstructure. We find the most suitable
hypothesis is to only increase the yield stress ($\sigma_{\rm y}$, plastic
component) based on the storage modulus of aged samples, while keeping the
viscous parameters ($K$ or $\eta_{\infty}$) constant. The work reveals
fundamental insight into rarely studied short-timescale flow conditions with
thixotropic effects. These results are important for applications such as fire
suppression or spray coating that involve complex fluids of varying degrees of
thixotropic aging.
