Transient contact behavior of aqueous polymer solution droplets with transparent hot solid

In the metal forming industry, quench hardening is commonly used to strengthen steel products by rapidly cooling hot materials. One of the typical coolants used in the quench hardening process is an aqueous polymer solution. During the hardening process, several phenomena occur simultaneously, namely, coolant boiling, separation of the polymer from the aqueous solution, and formation of a polymer-enriched layer on the solid surface. The hydrodynamics and heat transfer characteristics of the coolant during the cooling process are complex in nature and remain unclear. The main objective of this study is to develop an experimental understanding of the collision and contact behavior of aqueous polymer solution droplets with a hot substrate. This research will serve as a fundamental reference work for the process of quench hardening using spray cooling. To fulfill this objective, a three-directional flash photography technique was developed. A transparent sapphire prism was used to observe the transient contact behavior of droplets with a hot solid, whose temperature was varied from 300 to 600 °C. A solution of 10 wt% polyoxyethylene-polyoxypropylene glycol with an average molecular weight of approximately 20,000 was used as the test liquid. At a substrate temperature of 300 °C, jellylike polymer residue remained on the substrate. At 400 and 500 °C, a wet area appeared temporarily on the solid substrate soon after the droplet collision, but this area eventually disappeared because of the thermal decomposition of the polymer. At 600 °C, no wet area was seen. The lifetime of the temporary wet area decreased with an increase in the temperature of the solid but was almost independent of the impact inertia of droplets. In addition, in the case of the polymer solution droplet, the upper limit of the surface temperature for forming the wet area was around 580 °C, which was considerably higher than that in the case of a water droplet.