"Viscosity and Temperature
When a liquid heats up, its molecules become excited and begin to move. The energy of this movement is enough to overcome the forces that bind the molecules together, allowing the liquid to become more fluid and decreasing its viscosity. For example, when syrup is cold it has a high viscosity and can be difficult to pour. When heated in a microwave, the viscosity decreases and the syrup flows more freely.
What is Surface Tension?
Surface tension is what makes it possible to float a needle in a cup of water, or for water-skimming insects to glide across the surface of a lake. The molecules on the surface of a liquid are bound to the molecules beside and below them, but have nothing above them to balance out these attractive forces. Because of this imbalance, the molecules on the surface of the liquid will be drawn more strongly to those around it, creating a sheet of tightly bound molecules on the surface of the liquid.
Surface Tension and Temperature
As the temperature of a liquid increases, its surface tension decreases. When water heats up, the movement of its molecules disrupts the imbalanced forces on the surface of the water and weakens its sheet-like barrier of tightly bound molecules, thereby lowering the surface tension. This is why hot water is more effective when cleaning; its low surface tension allows it to more easily penetrate the fibres of a material like fabric and wash away stains."
I could have "Googled" to find out if there is some standard for the temperature of pool water for swim competitions - but reckoned I had learnt enough - our "feelings" are valid on this matter! Effort expended overcoming resistance = propulsion - and when we aim to get a "grip" on the water (i.e. the "catch") to expend that effort, we don't want our arms to slip through it too easily. This "slippage" when swimming is an interesting business - but could be too much for my little noddle to contemplate!
Although I've often wondered if, at the higher SPMs there was a lot of energy being wasted in slippage driving the arms through the water "as fast as bees wings" and effectively, "spinning the wheels". If you have any thoughts on this, please try not to "flummox" me with talk about the Reynolds number etc, unless it is essential to comprehension!
