  The aim of this study is to derive accurate models for quantities
characterizing the dynamics of droplets of non-vanishing viscosity in
capillaries. In particular, we propose models for the uniform-film thickness
separating the droplet from the tube walls, for the droplet front and rear
curvatures and pressure jumps, and for the droplet velocity in a range of
capillary numbers, $Ca$, from $10^{-4}$ to $1$ and inner-to-outer viscosity
ratios, $\lambda$, from $0$, i.e. a bubble, to high viscosity droplets.
Theoretical asymptotic results obtained in the limit of small capillary number
are combined with accurate numerical simulations at larger $Ca$. With these
models at hand, we can compute the pressure drop induced by the droplet. The
film thickness at low capillary numbers ($Ca<10^{-3}$) agrees well with
Bretherton's scaling for bubbles as long as $\lambda<1$. For larger viscosity
ratios, the film thickness increases monotonically, before saturating for
$\lambda>10^3$ to a value $2^{2/3}$ times larger than the film thickness of a
bubble. At larger capillary numbers, the film thickness follows the rational
function proposed by Aussillous \& Qu\'er\'e (2000) for bubbles, with a fitting
coefficient which is viscosity-ratio dependent. This coefficient modifies the
value to which the film thickness saturates at large capillary numbers. The
velocity of the droplet is found to be strongly dependent on the capillary
number and viscosity ratio. We also show that the normal viscous stresses at
the front and rear caps of the droplets cannot be neglected when calculating
the pressure drop for $Ca>10^{-3}$.
