Fluids are substances that continuously deform when a shear stress is applied, including liquids and gases. A fundamental property governing how a fluid behaves is its viscosity, which is its resistance to flow. Most liquids encountered daily, such as water and cooking oils, exhibit predictable flow characteristics regardless of how quickly they are stirred or poured. This article provides the precise method for creating a fascinating material—a fluid whose behavior changes dramatically based on how it is handled.
Clarifying Fluid Behavior
Viscosity measures a fluid’s internal friction, determining whether it flows like thin water or thick honey. For a majority of common liquids, this resistance to flow remains constant, even if the external force applied to it is varied. These are known as Newtonian fluids, meaning their viscosity does not change when they are subjected to different shear rates.
The material you are looking to create, however, belongs to a class of substances called non-Newtonian fluids. These materials do not follow that linear relationship, and their viscosity changes when a force or stress is applied to them. Some non-Newtonian fluids, like ketchup, become thinner when shaken, while others, like the one we will make, become thicker when subjected to sudden pressure. The instructions that follow are for creating this latter type, a shear-thickening non-Newtonian fluid.
Step-by-Step Guide to Shear Thickening Fluid
Creating this fluid requires only two common household ingredients: cornstarch and water. Gather a large mixing bowl, a measuring cup, and a box of cornstarch.
The most effective starting point is a ratio of approximately two parts cornstarch to one part water. For example, mix two cups of cornstarch with one cup of water. This ratio is a guideline, and adjustments are often necessary.
Pour the cornstarch into the bowl first, then slowly add the water a small amount at a time. Mixing this substance is labor-intensive, and using your hands is the easiest way to incorporate the ingredients. You will feel the resistance build immediately as the material begins to thicken.
If the mixture seems too crumbly or dry, add a tablespoon of water. If it flows too easily, incorporate more cornstarch until the desired consistency is achieved. The final product should flow and drip when no pressure is applied, yet be difficult to stir quickly.
Test the fluid by gently stirring it with a finger, which should be easy. Next, try tapping the surface quickly or plunging your finger into it with force. If the ratio is correct, the substance should resist the quick impact, feeling firm and solid.
Explaining the Physics of the Mixture
The material’s behavior is categorized as shear thickening, meaning its viscosity increases when a quick force is applied. This fluid is technically a suspension, where tiny, solid cornstarch particles are dispersed within the liquid water. The particles are suspended in the medium, not dissolved.
When the fluid is handled gently, the water acts as a lubricant, allowing the cornstarch particles to slide past one another easily. This permits the suspension to flow smoothly, behaving like a typical liquid. The particles have enough space and time to rearrange themselves around the slow movement.
The scenario changes when a sudden, high-force impact is introduced, such as a quick tap or punch. The rapid pressure forces the suspended particles closer together, jamming them temporarily into a packed state. The water, unable to escape quickly enough, becomes trapped between the compressed particles, creating a semi-rigid structure that resists the force.
This momentary solid-like behavior is caused by the frictional contact between the particles once they are pushed together, which increases the material’s internal resistance. As soon as the external force is removed, the packing breaks apart, and the cornstarch particles relax back into their suspended state, allowing the material to flow freely again like a liquid.