Dynamics of formation and oscillation of non-spherical drops Hiranya Deka a , Pei-Hsun Tsai b , Gautam Biswas a,⇑ , Amaresh Dalal a , Bahni Ray c , An-Bang Wang b a Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India b Institute of Applied Mechanics, National Taiwan University, Taipei 10617, Taiwan c Department of Mechanical Engineering, Indian Institute of Technology Delhi, Delhi 110016, India highlights  The aspect ratio of drops continuously increases with the increase in Weber number.  Increased viscosity of the drop fluid damps the velocity gradient inside the drop.  The shape of a drop is either prolate or spherical at the incipience of pinch-off.  A non-spherical drop oscillates while falling through a surrounding medium.  The oscillatory motion of the drops is damped by viscosity. article info Article history: Received 30 November 2018 Received in revised form 4 March 2019 Accepted 5 March 2019 Available online 8 March 2019 Keywords: Drop Pinch-off Surface tension Aspect ratio Oscillation abstract Present study focuses on the shape of a drop during pinch-off and how the shape of a drop is affected by the flow parameters and different physical properties of the fluids. Both experimental and numerical investigations are performed to unveil the mechanism of non-spherical drop formation and the oscilla- tory motion of a drop after pinch-off. Numerical simulations are performed over a wide range of density ratios (0.001–0.9) and viscosity ratios (0.01–10) ranging from a gas-liquid system to a liquid-liquid sys- tem. The deformation of a drop at the incipience of pinch-off depends on the internal stress distribution within the drop. A larger velocity gradient inside the drop culminates in a stronger shear force within the drop leading to a non-spherical drop during pinch-off. We reveal that the shape of a drop remains either in prolate or in spherical form during pinch-off. A drop undergoes prolate-oblate-prolate oscillation while falling through the surrounding medium. The oscillatory motion is generated due to the local capillary pressure that develops across the drop surface because of the deformed shape of the drop after pinch-off. Present investigation reveals that the deformation of the drop because of the downward pull generated near the pinching region makes the drop shape non-spherical after pinch-off and initiates the oscillatory motion of the drop. The oscillation of a drop reduces substantially at higher relative strength of the viscous force. Ó 2019 Elsevier Ltd. All rights reserved. 1. Introduction Drop formation is a ubiquitous natural phenomenon having numerous applications in science and technology. It bears far- reaching implications in a variety of engineering processes such as surface tension measurement by the drop weight method (Harkins and Brown, 1919; Yildirim et al., 2005), separation and extraction processes (Heideger and Wright, 1986), functioning of sprays (Eggers and Villermaux, 2008; Reitz and Beale, 1999), inkjet printing (Shield et al., 1987), cell and organ printing (Wilson and Boland, 2003), to name a few. In addition, drop formation has been a topic of scientific research interest for more than a century due to the richness of the underlying physics (Harkins and Brown, 1919; Tate, 1864; Lord Rayleigh, 1899). Drop formation can be broadly classified into two categories, namely dripping and jetting. At a low flow rate, the drops form periodically one after another, which is commonly known as the dripping regime. With the increase of flow rate, a transition into the jetting mode is observed where the inertia force becomes dom- inant. In the jetting mode, a long jet is formed which is inherently unstable and breaks up forming several drops. Many studies have been performed experimentally, theoretically and numerically to uncover the fascinating aspects of dripping (Wilson, 1988; Eggers https://doi.org/10.1016/j.ces.2019.03.008 0009-2509/Ó 2019 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: gtm@iitg.ac.in (G. Biswas). Chemical Engineering Science 201 (2019) 413–423 Contents lists available at ScienceDirect Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces