What is the most efficient way to stir to dissolve something in water: with circular motions, chaotic ones, fast or slow?
Karen Tansley
Milnthorpe, Cumbria, UK
Advertisement
My grandad, a keen gardener, swore by steady, figure-of-eight motions through the liquid, making sure whatever you were stirring with reached the bottom of the vessel (usually a watering can). Many of the garden feeds he used were granular and thorough mixing was important.
I have used this method since he taught it to me and it continues to work consistently. I even use it in my cups of tea.
@ScienceAmbass
via Twitter
Surely it depends on what solid is being dissolved and the concentration? A dash of sugar in tea needs just a quick swirl around, but Cup a Soup or hot chocolate requires specific excavation of the undissolved gloop around the edges at the bottom.
Simon Dales
Oxford, UK
The best way to mix is to zigzag the stirrer. The objective is to expose as much surface area of the liquid to itself as possible.
A zigzag will stretch and fold the greatest volume. If you stir circularly, the liquid just follows the stick so can’t mix much. You can observe this with sugar and water in a cup. Also, do this too fast and it “centrifuges” over the sides.
Zigzag movements only wiggle a small amount of the liquid at any one time, so reducing the sloshing. This is important when mixing liquids containing suspended particles such as paint, because the carrier liquid is so runny and sloshes at the first opportunity. Tahini is tricky to mix too: the oil sloshes easily, but the solids put up a fight. A long, thin stirrer is best to get the solids off the bottom of the vessel and rip them apart.
Mark Cargill
Alcester, Warwickshire, UK
Mixing liquids effectively seems simple, but when it comes to tea, which is the best method?
Mixing or agitating a liquid means creating vortices in it. These then homogenise the component parts of the liquids and any dissolved solids, such as sugar. Another element is rotational speed to keep the vortices going for longer, giving better mixing. A single stroke either up and down or side to side is quick but inefficient. With a figure-of-eight motion, the majority of the liquid is touched by the spoon, creating a rotation of the liquid plus those all-important vortices.
This method works up to a point, but with more viscous fluids, like paint, additional movements, such as an occasional diagonal stroke across the figure-of-eight, are needed to ensure efficient mixing.
Mark Wareing
Ashbourne, Derbyshire, UK
The first stage of successfully dissolving a fine powder in water is adding the powder in a way that minimises the formation of lumps. This is best done by using one hand to stir with a spoon in a circular motion at about 200 to 250 revolutions per minute to form a liquid vortex into which the powder is poured with the other hand.
The dispersed powder will initially circulate at the base of the container, reducing the solubility of undissolved powder. Therefore, at this stage, the stirring should be changed to a more turbulent system to mix the powder more evenly throughout the liquid and increase the rate at which it dissolves. Slower stirring with the spoon (at 60 to 70 cycles per minute) in a figure-of-eight pattern that touches the base of the container can be used.
Powder particle size is key. A relatively coarse powder such as caster sugar can easily be stirred in without lumping, hence the vortex stage is less important. In contrast, a fine powder such as icing sugar – which might be expected to dissolve more quickly – needs dispersing with the vortex method to reduce lumping.
Kim Heywood
via Facebook
Well, there is “efficient” and there is “less messy”. I prefer not to slop stuff all over the table. It may be slower, but there is less mess to clear up afterwards.
[Ed – if you want to take stirring to the next level, the best way to mix two liquids is using two pentagon-shaped stirrers, moving in a jagged half circle, according to a 2022 study]
To answer this question – or ask a new one – email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
91av retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.