Dilute polymer solution drops impacting on heated surfaces: New impact morphologies and impact regime maps

Abstract

The impact morphology of dilute polymer solution drops on a heated surface is studied experimentally by means of high-speed imaging, with respect to the following parameters: surface temperature; impact Weber number; polymer concentration; polymer molecular weight. In addition to impact morphologies observed in Newtonian drops (deposition, rebound, secondary atomisation and breakup/splashing), three new impact regimes have been identified: (i) a single satellite droplet ejected in the direction of bouncing but tethered to the main drop by a thin liquid filament; (ii) a splashing-like behaviour (semi-splashing), where the rim instability generates satellite droplets tethered to the lamella by thin liquid filaments; (iii) a spray-like behaviour (semi-spray), where a fine secondary atomisation generated upon impact is quickly absorbed back into the drop globule. Experiments were carried out using drops of aqueous polyethylene oxide (PEO) solutions, with mass concentrations of 100 ppm, 200 ppm and 400 ppm, and PEO molecular weights of 2 MDa, 4MDa, and 8MDa. The impact morphology on a polished aluminium surface with temperatures ranging between 160°C and 400°C was investigated for impact Weber numbers between 20 and 170, taking side view images of impacting drops at a rate of 1,000 frames per second. KeywordsDrop Impact; Heated Surfaces; Polymer Solutions; Impact Regime Map. IntroductionThe dynamics of liquid drops impinging on a heated surface is a phenomenon of interest in many areas of engineering, including spray cooling, inkjet printing for advanced manufacturing applications, quenching of alloys in the steel industry and nuclear reactor safety. Despite several decades of research, this phenomenon is understood only to a limited extent and requires further research; specifically, the development of impact regime maps (IRMs) is of practical interest because they provide a consolidated understanding of the impact behaviour and represent a tool of practical use in the design of nozzles, print heads, etc. A quantitative and comprehensive IRM also presents an important test case for the development and validation of theoretical models describing specific features, such as transitions between different impact regimes. Currently, an extensive body of literature on the behaviour of Newtonian drops impacting on heated surfaces exists. The first IRM produced [1] identified most of the impact regimes, however lacked clear quantitative description on the initiation and extent of these regimes. This work was soon followed by a detailed investigation that allowed production of three distinct IRMs corresponding to Weber numbers of 20, 60 and 220 and for surface temperatures ranging from 100° to 300°C [2]. Although this work included substantial information on the spreading behaviour of a drop and its heat transfer characteristics, the IRMs were far from being fully comprehensive, particularly lacking information on all possible impact regimes that can be realised as well as the transitional boundaries. This was partly ...