Experimental study on the electrohydrodynamic deformation of droplets in a combined DC electric field and shear flow field

Abstract

• Electrohydrodynamic deformation of droplet in combined electric field and shear flow is studied. • Internal flow of the deformed droplet is quantitatively examined via the DPIV method. • Competition of actions from electric field and shear flow on droplet deformation is clarified. • Prediction model is modified and demonstrated to quantitatively represent droplet deformation. A visualization experiment was conducted to investigate the electrohydrodynamic deformation of droplets in a combined DC electric field and shear flow field. Detailed experimental data on both the transient and steady droplet deformation parameters ( D ) and orientations ( ϕ d ) are provided at R > S and R < S ( R : conductivity ratio; S : permittivity ratio) under different electric field and shear flow field combinations. The internal flow characteristics of the deformed droplet were also examined via the digital particle image velocimetry (DPIV) method. Due to the competition of the extensional component (EC) and the rotational component (RC) of these two fields on the droplet, the response of ϕ d is faster than that of D when an electric field is combined with a shear flow. Additionally, under the competition of the EC and RC at R > S and R < S , the steady-state D and ϕ d values exhibit distinct variations. In particular, surface charge convection plays a non-negligible role in enhancing and reducing droplet deformation at R > S and R < S , respectively. In addition, an asymmetric vortex forms inside the deformed droplet in the combined fields, and its velocity is lower under R > S and higher under R < S than in pure shear flow. The available prediction models use the experimental data to predict D , and a modified prediction model is proposed for improving the prediction accuracy of ϕ d .