Huihui Xia

We carried out experimental and numerical investigations on the axisymmetric spreading evolution of dynamic spin coating with a single drop of ethanol. The results show that the dynamic spreading process consists of two stages: inertial spreading stage and centrifugal thinning stage. These two stages are connected by a transient state in between characterized by the minimum contact line moving velocity. The Weber number determines the spreading in the first stage, similar to the case of the impact of a drop on a static substrate. The rotational Bond number has a marginal effect on the inertia spreading and the radius at the transient state. In the centrifugal thinning stage, the rotational Bond number dominates the flow while the effect of the Weber number is negligible.

Whipping is a peculiar nonaxisymmetric instability experienced by electrified liquid jets and utilized in electrospinning to produce fine polymer fibers; however, it manifests itself in a chaotic and uncontrollable fashion. We impose two perturbations in the form of small mechanical oscillations, orthogonal with each other, to alter and curb instabilities of electrified liquid jets. The oscillations cause transverse positioning disturbance to the nozzle and therefore the initial positioning of the jet. The effects of parameters including the excitation frequency, amplitude, and phase difference of the imposed perturbations and the axial electric field are investigated. The steady helicoidal whipping structure with a linearly growing lateral amplitude in air has been demonstrated, offering a feasible solution for electrospinning to fabricate fibers of uniform thickness. Furthermore, the superposition of two sinusoidal orthogonal perturbations enables deposition of complex patterns, such as Lissajous curves.

Whether an electrified jet breaks up while the jet maintains a straight line (varicose), or alternatively, the jet trajectory becomes chaotic (whipping), depends on the competition of the interfacial forces. Starting from the competition of normal stresses on the jet surface, we derive scaling laws in different electro-hydrodynamic operating regimes as a function of fluid properties and the flow rate. The onset of whipping occurs when this scaling function reaches a threshold, which is independent of the electric field strength. However, experimental evidence indicates that this onset condition applies only when the viscosity and electrical conductivity of a liquid are small enough. As a result, we further introduce a general parameter to incorporate viscous effects into the scaling law. A unified threshold value for this parameter is found through a substantial number of experiments for liquids having a wide range of properties and under a wide variety of operational conditions of flow rate.