Around March 2022, a major ingredient change was made to a Japanese dishwashing detergent (Cucute/Kyukyutto). The old Cucute disappeared from store shelves and the new Cucute began to appear on store. The new Cucute has changed both the ingredients and the surfactant ratio (reduced: from 37% to 32%). It became necessary for me to analyse the new Cucute.
My first thought was to compare the color profiles of the old and new Cucute.
I prepared two solutions: I added 2 teaspoons of 1% PEO solution to 1,600 ml of water and then poured it into two containers (800 ml each). I added 40 g Old version Cucute to one and 40 g New version Cucute to the other.
I set up the apparatus for the experiment, referring to Edward's Longevity Test experimental setup. However, I did not have a suitable black background against which to observe distinct colors.
I was at a loss for a while, but when I looked at the wall of the room, I suddenly noticed the shadow of a soap bubble projected by the lighting onto the white wall. The projection showed the movement of the flowing liquid like a gastric radiograph, and I could also see a slight hint of color in the outline of the bubble.
I decided to record this projection.
A video of the projection of the old version Cucute soluion and the new version Cucute solution is shown below. What is recorded is not the bubble itself, but the projected view of the bubble.
The bubble is placed over the mouth of the plastic bottle. Liquid is pulled from the bottle mouth along the film of bubble, and upflow to the top of the bubble.
In the old version, the water flow reaches the zenith, whereas in the new version, it loses momentum at the midpoint and stops. Eventually, the new version loses moisture at the zenith and the film becomes thinner.
In the new version, some force seems to be preventing the upward motion.
In the next video, the mouth is set up facing down.
In the old version, the entire edge of mouth supplies liquid evenly and the liquid moves across all over the film; in the new version, the film surface is static, and motionless, and the water flows down in some thick streaks from particular points.
(The experiment was conducted over two days, with 20 shots of each version, old and new. Each time with consistent results.)
I had an idea about this. When I previously tried the new version Cucute I noticed a large amount of dripping from the bottom of the bubble. When I looked at the bubble hanging from the wand, I saw water streaking along the sides of the bubble and falling to the bottom. I blew bubbles as usual, and they fell straight to the ground. They also felt more fragile to me than before. I thought maybe too much water was in the bubbles so I reduced the water from the recipe and increased the detergent but saw no improvement.
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What puzzled me in this experiment was that the new version Cucute with a lower surfactant rate resulted in a thinner bubble than the old version Cucute with more surfactant. (https://youtu.be/eqBpi3RVaPM)
Intuitively, if the same amount of water and the same amount of detergent are used, the new version with less surfactant should have produced a thicker film. The fact that when the bubbles were blown they did not float but fell immediately also suggested that its film was thick. But result of projection experiment was the opposite (the results have been consistent). It confused me.
However, after looking at the projection repeatedly, I began to suspect that another factor (a certain characteristic of new version Cucute) was causing this, other than the surfactant content.
I suspect that new version Cucute has the property of strongly (and rapidly) ejecting water. In the movie, the bottom half of both bubbles are almost the same thickness, but the zenith of the bubble is extremely thin only in the new version. The strong ejection of water (falling along gravity) may have prevented water from being pulled up.
When I blow the bubble, the new version Cucute solution results in more water in the bubble. Copious dripping drags the bubble to the ground.
The New version Cucute is now "completely different" from the old version Cucute (with the same brand name). It is no longer possible to reproduce the old version Cucute solution with today's materials.
As far as I analyze the bubble from the viewpoint of projection, I have no choice but to conclude so.
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Kao's website advertises that the new CuCute is designed to make foam fall off more quickly than before when water is poured over washed dishes. This advertising seems to support my experiment.