Dishwasher detergent formulations contain components which dictate the chemical environment (pH, wettability, solubility) of cleaning solutions. The role of these factors, as well as temperature, in the mechanisms controlling the cleaning of a baked heterogeneous food soil from stainless steel substrates was studied using a combination of fluid dynamic gauging, real-time imaging, millimanipulation, and solution analysis techniques. The extent of swelling, which is known to affect cleaning, was determined in situ using a fluid dynamic gauging (FDG) device developed by Wang et al. (2017). A new FDG configuration was developed which enabled measurement of swelling soon after immersion, allowing measurement of initial hydration. These studies were corroborated using a commercial point-light source confocal thickness measurement device. The onset and extent of swelling depended primarily on the solution pH.
At temperatures above 35 ᵒC swelling was followed by the liberation of mobile fat present within the soil. Monitoring of droplet evolution allowed the growth and detachment of the oil droplets to be quantified and modelled. The rate of total carbonaceous material released from the soil was studied in separate tests using both stagnant and flowing solutions. The onset and volume of oil released was chiefly determined by the solution temperature and concentration of surfactant. Oil release was not directly related to deposit strength.
In millimanipulation the force imposed on a blade being passed through the soil layer is measured. The device was modified to allow submersion and flow of cleaning solution across the sample so that the effect of contact time with the reagent(s) could be studied. The force required to remove the soil changed noticeably after a critical soaking time, from an almost constant value to one which decayed with time. The critical soaking time depended on the temperature, pH, and composition of the cleaning solution and in many cases was associated with a transition from cohesive to adhesive breakdown. This transition occurred on similar timescales to the swelling of the soil. Some agents promoted faster adhesive breakdown.
Sinner’s circle is classically used to describe the intentions between temperature, formulation, time and mechanical forces in cleaning. The different techniques allowed these to be quantified, particularly in terms of timescales. The cleaning mechanism was broken into two parallel processes: (I) the hydration and swelling of the soil layer after exposure to aqueous solutions followed by the dewetting and displacement of oils and fats from within the soil structure towards the soil-solution interface, and (II) the ingress of solution at the soil-substrate interface, weakening the adhesive forces attaching the soil to the substrate. Temperature, pH, and surfactant type were demonstrated to act each process to a different extent, influencing the timescales of cleaning.