Increasingly sophisticated weapons continue to pose new threats for soldiers in the frontline. Thus, efforts are underway to develop more protective and reliable body armor. Researchers at the United States Army Research Laboratory recently developed shear thickening fluid (STF), a lightweight and flexible protective material that can do the work of its heavier and bulkier conventional counterparts. The discovery and subsequent implementation of such materials has increased the demand for armor that is not only durable, but also allows for heightened maneuverability and operational effectiveness. According to Eric Wetzel, who led the military research team, “the goal of the technology is to create a new material that is low-cost and lightweight, and that offers equivalent or superior ballistic properties as compared to current Kevlar fabric but has more flexibility and less thickness” (1).
A Nanoscale Discovery
Shear thickening fluid, colloquially known as “liquid body armor”, was developed in 2002 at the United States Army Research Laboratory in Adelphi, Maryland. Two scientists – Norman Wagner, a professor of chemical engineering at the University of Delaware, and Eric Wetzel, a staff member at the laboratory – led the development team. The application for US Patent No. 7,226,878, the patent for “Advanced Body Armor Utilizing Shear Thickening Fluids,” was submitted in 2003 and issued four years later (2). Wagner and Wetzel were subseqently awarded the Paul A. Siple Award, the Army’s highest award for scientific achievement.
Several properties of shear thickening fluid make it desirable for use in protective clothing. Most significantly, STF behaves as a liquid until it is exposed to mechanical stress. At that point, within a matter of milliseconds, it hardens into a solid. Thus, when there is no threat to the wearer’s safety, he or she experiences little impairment in flexibility or range of motion. However, a ballistic or penetrative threat instantaneously activates the protective features of the armor (3).
Shear thickening fluid is an application of the rapidly growing field of nanotechnology. STF is a colloid, a mixture in which one substance is dispersed throughout another (4). Liquid body armors employ an STF that consists of 450 nm silica nanoparticles suspended in polyethylene glycol or ethylene glycol, both liquid polymers. The silica particle concentration must be between 55 and 65 percent by volume for optimal protective capabilities (5).
When STF is in liquid form, the weak molecular interactions between the silica particles permit them to move around freely in the liquid polymer without binding to one another. However, a ballistic or penetrative strike to the material (because the energy of impact is much greater than the energy between the metal particles) forces the particles to temporarily assemble into hydroclusters – long irregularly shaped chains of molecules. The hydroclusters subsequently overlap to form a mesh-like structure, which dramatically increases the viscosity of the liquid (6). As soon as the energy from the mechanical stressor disappears, this process reverses itself, and the substance returns to a liquid state.
Shear thickening fluid is considered to be a “non-Newtonian” fluid, a fluid that behaves in a way that contradicts Newton’s original theories. Newtonian fluids have a constant viscosity unless exposed to changes in temperature or pressure. Non-Newtonian fluids, in contrast, experience changes in viscosity in situations in which most fluids would not. In the case of STF specifically, its viscosity is dependent on shear stress (6). However not all non-Newtonian fluids exhibit the same physical changes in response to stress – while stress increases the viscosity of STF, it decreases the viscosity of shear thinning fluids, such as paint.
Unique Properties Allow for Increased Protection
The concept of “liquid body armor” can be misleading – there is in fact no external, visible liquid layer. Rather, shear thickening fluid is used to reinforce conventional forms of body armor such as Kevlar. While Kevlar has proven to be effective, 20 to 40 layers of its high-strength aramid fibers are required to stop a bullet, and ceramic tile inserts are necessary in high-threat situations (5). This form of body armor protects the wearer well but decreases flexibility and hinders performance.
The data from a study conducted by the United States Army Research Laboratory showed that four layers of Kevlar impregnated with STF could dissipate the same amount of energy as 14 layers of neat Kevlar. Furthermore, treating the Kevlar with STF adds little extra weight or thickness. Treating four layers of neat Kevlar with STF increases its mass by 2.9 g and its thickness by 0.1 mm. In general, saturated Kevlar can be 45% thinner than neat Kevlar without posing a threat to the wearer’s safety (5).
In addition to increasing the wearability of conventional body armor, STF saturation also reduces the pain resulting from a bullet strike. This is due to STF’s increased energy dissipation capacity. According to BAE Systems, the company currently manufacturing this product, liquid armor disperses mechanical stress over a wider area, restricting the depth of penetration (7). For the lowest impact velocities tested, the saturated Kevlar was never penetrated (5).
STF is applied to conventional ballistic fabrics, such as Kevlar, by diluting the liquid with ethanol, applying the diluted mixture to the fabric, then evaporating the ethanol. Fabrics saturated with STF retain all the desirable properties of unsaturated fabrics.
While STF-incorporated armor is not currently ready for combat, its inventors believe that it shows great promise. They predict that STF-saturated fabrics will soon supplement, if not entirely replace, conventional forms of body armor. In addition, this technology will likely be applied to shields, sportswear, and protective clothing for police officers, prison guards, and ambulance crews (7). There is also potential for its use in gloves for medical professionals in order to prevent needlestick injury and infection.
When asked about the prospects of this technology, Wetzel replied, “The sky’s the limit.” He continued, “We would first like to put this material in a soldier’s sleeves and pants, areas that aren’t protected by ballistic vests but need to remain flexible. We could also use this material for bomb blankets, to cover suspicious packages or unexploded ordnance. Liquid armor could even be applied to jump boots, so that they would stiffen during impact to support soldiers’ ankles” (8).
However, by no means are the applications of shear thickening fluid limited to protective clothing. In the oil industry, shear-strengthening materials are used in combination with other drilling fluids to ‘patch’ wells in order to control some forms of blowouts (9). Additionally, engineering students at Case Western Reserve University in Ohio used Oobleck, a shear thickening fluid made with cornstarch and water, to fill potholes. The students believed that STF was the ideal material for this project because of its ability to both “fill irregularly-shaped depressions” and to “become rigid in the presence of passing automobiles” (10). BAE systems also plans to “further engineer” STF to ensure high levels of thermal stability and moisture resistance, properties which Kevlar is argued to lack (5).
STF’s wide-ranging applications demonstrate the innovative employment of its non-Newtonian properties. The extent of its versatility is bounded only by our creativity, and it will take new minds to expand upon STF’s implementation in our society, both as a protective material and beyond it.
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2. A. Guerrero, Liquid Body Armor. Available at http://www.nisenet.com/sites/default/files/catalog/uploads/8379/liquid_body_armor.pdf (5 September 2013).
3. Shear Thickening Fluid Showing Promise for Increasing Soldier Protection (8 December 2010). Available at http://www.arl.army.mil/www/default.cfm?video=9 (4 September 2013).
4. Colloid Definition. Available at http://www.merriam-webster.com/dictionary/colloid (21 September 2013).
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6. D. Price, Shear-Thickening Fluid (10 May 2012). Available at http://physics.wooster.edu/JrIS/Files/Price_Web_Article.pdf (21 September 2013).
7. Flexible Friend on the Front Line. Available at http://www.baesystems.com/home?_afrLoop=127439853666000 (7 September 2013).
8. T. Johnson, Army Scientists, Engineers Develop Liquid Body Armor (21 April 2004). Available at http://www.military.com/NewsContent/0,13319,usa3_042104.00.html (21 September 2013).
9. McCreary, Shear Fluid Paper. Available at http://paultheengineer.com/print/shearfluidpaper.pdf (19 September 2013).
10. B. Brownell, A Practical Use for Oobleck (2013). Available at http://www.architectmagazine.com/blogs/postdetails.aspx?BlogId=mindmatterblog&postId=107817 (19 September 2013).