On the deformation of a shear thinning viscoelastic drop in a steady electric field

Abstract

The deformation of viscoelastic drops under electric fields plays a crucial role in applications such as microfluidics, inkjet printing, and electrohydrodynamic manipulation of complex fluids. This study examines the deformation and breakup dynamics of a linear Phan-Thien-Tanner (LPTT) drop subjected to a uniform electric field using numerical simulations performed with the open-source solver Basilisk. Representative combinations of conductivity ratio ($\sigma_r$) and permittivity ratio ($\epsilon_r$) are chosen from six characteristic regions of the ($\sigma_r$, $\epsilon_r$) phase space, $PR_A^+$, $PR_B^+$, $PR_A^-$, $PR_B^-$, $OB^+$, and $OB^-$. In regions where the first- and second-order deformation coefficients have the same sign ($PR_A^-$, $PR_B^-$, $OB^+$), the LPTT drops exhibit deformation dynamics that negligibley deviate from the Newtonian behavior. In the $PR_A^+$ region, drops deform into prolate spheroidal shapes below a critical electric capillary number and transition to stable multi-lobed shapes or breakup beyond this threshold. Increasing elasticity of drop opposes the deformation, thereby reducing deformation and increasing critical $Ca_E$ with the Deborah number ($De$). In the $PR_B^+$ region, drops form prolate shapes below critical $Ca_E$ and develop conical ends above it. The steady-state deformation exhibits a non-monotonic dependence on $De$, increasing at low $De$ and decreasing at higher values. A similar non-monotonic variation is also observed in critical $Ca_E$. In the $OB^-$ region, LPTT drops attain oblate shapes below critical $Ca_E$ and undergo breakup beyond it. The deformation magnitude shows a non-monotonic variation with $De$, increasing initially and decreasing at higher elasticity.