2.1 Quantification of mixing time
The dual indicator system for mixing time (DISMT) technique was used to
quantify mixing time in this work. The method used was the same as that
reported by Li, Ducci, & Micheletti, 2019. The DISMT technique was
first developed by Melton, Lipp, Spradling, & Paulson, 2002 and is
based around pH dependent colour changes. As the name implies, two pH
indicator solutions - thymol blue (Fisher Scientific, Loughborough, UK)
and methyl red (Fisher Scientific, Loughborough, UK) were used for
visualisation of pH change with the fluid appearing red in acidic
conditions, yellow at neutral pH and blue in basic conditions. The pH of
the working reagent was adjusted until it became neutral (i.e. yellow).
After filling the vessel with the working reagent, a stoichiometric
amount of 0.075M hydrochloric acid (HCL) (Sigma-Aldrich, St. Louis,
Missouri, United States) was added to make the colour transition from
yellow to red. Thereafter, an equal amount of 0.075M sodium hydroxide
(NaOH) (Sigma-Aldrich, St. Louis, Missouri, United States) was added to
neutralise the acid and return the colour to yellow. The mixing process
was recorded using the iCube (NET GmbH, Germany) high speed camera upon
the addition of base. Mixing can be visualised since only regions that
are well mixed appear yellow. Each condition was repeated five times to
reduce statistical error and the average value was used as the mixing
time. A universal MATLAB (MathWorks, Massachusetts, United States)
script was used to compute the mixing time. The image processing method
was the same as that used by Li et al., 2019.