# What are Dilatants?

By Nick Gilbert

## Introduction

A Diltant, often referred to as a 'shear thickening' material portrays some extremely interesting material properties; it's viscosity actually increases with the applied rate of shear strain. Therefore, in simple terms, the harder and faster you push it, the more resistance you feel. This curious behavior means that Dilatant materials and fluid's are Non-Newtonian. The following graph highlights this relationship:

Figure 1. Non-Newtonian Behavior - Dilatants

Having Non-Newtonian properties allows people to use Dilatants in some extremely innovative and exciting ways. They are also great fun to experiment with. Just take a look at this great Video from the Guys at Boston University.

Figure 2. Non-Newtonian Pool Party - Boston University

Run Time - 2:42mins

## What are Shear Thickening Materials / Dilatants?

Shear Thickening Fluid's (STFs) are examples of Non-Newtonian fluids, a rare set of materials and fluids who's behavior deviates from Newton's law - 'Every action has an equal and opposite reaction'.

A Dilatant material is a material that features an increase in viscosity and can even set to a solid as a result of deformation by expansion, pressure or agitation. You might think that these types of materials are extremely rare and are only available for Scientists to play with but the truth is, Dilatants can easily be created at home using simple ingredients such as Corn Flour, Water and even Sand and water etc. Child's Silly Putty is also a great, everyday example of Dilatant behavior. How can something be squashed easily in a child's hand but when you throw against a wall will also rebound like a bouncy ball?

Shear thickening behavior is observed when the force applied changes the structure within the fluid. For example wet sand / slurry on a beach can often exhibit dilatant properties. Whereby, if you stand still you might sink into the wet sand but if your were to run across the same bit of sand your feet will not sink in as much. For many this is counter-intuitive. For example; how can a liquid behave like a solid when it is hit hard but then return to behave like a liquid almost immediately after? This amazing behavior has unique applications for which industry is continually studying and developing for commercial use.

## What Makes a Fluid a Dilatant?

Dilatant or Shear Thickening behavior is typically observed in fluids with a high concentration of small solid particulate is suspended within a liquid. Behaving like a true fluid under low shear stress conditions, Dilatants then exhibit a transition to a more solid like behavior when a greater shear stress or force is applied. The greater the force (shear) applied to a dilatant material, the more resistance you will feel. When subjected to extremely high levels of shear stress dilatant materials can quickly become very rigid.

Dilatant behavior is controlled by factors such as particle size, shape, distribution and the ratio of particle density suspended within the fluid. Shear thickening behavior occurs when a suspension transitions under applied stress from a stable state to one that is in a state of flocculation (unstable).

Dilatancy within a colloidal suspension occurs when the amount of shear stress exceeds the ability of the inter-particle forces to maintain their original structure. At this point the fluid enters a state of Flocculation and it will begin to behave more like a solid. In Colloidal suspensions, the shear rate at which the fluid transitions from Newtonian to Non-Newtonian, or shear thickening behavior is known as the critical shear rate.

## Shear Thinning Fluids / Materials

Interestingly there are also materials and fluids that exhibit Shear Thinning properties. Where; as more stress is applied, the viscosity increases, ie. the resistance you feel becmes less. Pseudoplastics also portray these characteristics.

## Rheopectic Materials - Not to be Confused with Dilatants

Rheopecty is a rare property of some materials / fluids that is also Non-Newtonian behavior but is distinctly different to Dilatant behavior. Rheopectic fluid viscosity has a time dependent function. For example, the viscosity of a fluid changes with an applied force over time. Therefore, the longer a fluid experiences an applied force, the higher the resultant viscosity.

Rheopectic fluids are predominantly used for lubricants, Gypsum pastes and printer inks although many applications and potential applications exist in a whole host of interesting industry's and markets. This author also believes that there may well be applications in the transport of highly sensitive goods and instruments.

The opposite behavior to Rheopecty is called Thixotropy, which is far more common. In this example, fluids become less viscous the longer a shear force is applied.

## Current and Future Applications of Dilatants

Materials and Fluids that exhibit Non-Newtonian behavior such as Dilatants offer a diverse range of potential opportunities and future applications.

Potential Applications of Dilatants:

• Shock absorption Systems
• Automotive Suspension - Magnetic particles suspended
• Impact Stress Cushioning - Sport / Athletics
• Accident damage and injury mitigation - Transport
• Impulse Distribution Systems
• Smart Body Armour

Figure 3. An interesting potential future application: Smart Body Armour Introductory Video

Run time - 2:00mins