Buzz
While ancient and modern body armor share the common goal of providing protection against weaponry, both are often characterized by their weight, bulk, and lack of flexibility.The fundamental principle behind body armor has remained largely unchanged over millennia. Initially, armor serves two main functions: it prevents weapons or projectiles from penetrating the body, and it disperses the energy from the weapon, reducing the damage from the impact. Although not foolproof in every scenario, armor generally offers substantial protection, particularly when designed for specific threats, reducing the risk of serious injury or fatality.
As weapons advanced over time, so too did the development of stronger and more sophisticated body armor. Despite these improvements, modern armor still shares some characteristics with its ancient counterparts. Whether crafted from metal plates or woven fabrics, body armor remains bulky and heavy. Many designs are rigid, restricting movement in certain areas such as the arms, legs, and neck. To accommodate movement, medieval plate armor included gaps and joints, while modern armor often focuses on protecting only the torso and head.
One of the most promising innovations in body armor is a new, lightweight and flexible design that incorporates liquid. Although it's not yet ready for use in combat situations, early laboratory findings suggest that liquid body armor could one day replace or complement traditional bulky vests. This development could provide enhanced protection for the limbs, allowing soldiers, law enforcement, and others to move more freely while still staying safe.
The two main types of liquid body armor currently under development both utilize a base layer of DuPont Kevlar, a material widely recognized for its use in bulletproof vests. When a bullet or shrapnel strikes Kevlar, the layers disperse the impact across a wide area. Additionally, the bullet stretches the Kevlar fibers, absorbing energy and decelerating in the process. This concept mirrors the function of a car airbag, which spreads the impact and reduces the force on the driver’s torso during a collision.
While Kevlar is a fabric, it doesn't have the same flexibility or drape as regular clothing. It typically requires between 20 and 40 layers of Kevlar to halt a bullet, creating a relatively rigid and heavy stack of material. Even without extra ceramic inserts for additional protection, a vest can weigh over 10 pounds (4.5 kilograms).
However, two distinct fluids can enable Kevlar armor to use far fewer layers, resulting in a much lighter and more flexible design. Both of these fluids share one key feature — they react powerfully when triggered by an external force. Let’s explore the composition of these liquids and the reasons behind their reactions.
Shear-thickening Fluid
Shear-thickening fluid used in liquid body armor
Photo by Sgt. Lorie Jewell/ courtesy U.S. ArmyThe term "liquid body armor" can be somewhat misleading. For some, it might evoke an image of a fluid layer between two solid surfaces. However, both of the liquid armor types currently being developed function without any visible liquid layer. Instead, they rely on Kevlar that is saturated with one of two different fluids.
The first fluid is a shear-thickening fluid (STF), which acts like a solid when exposed to mechanical stress or shear. Essentially, it behaves like a liquid until it is struck or forcefully disturbed. Upon impact, it solidifies in mere milliseconds. This is the opposite of a shear-thinning fluid, such as paint, which becomes thinner when stirred or shaken.
To understand shear-thickening fluid, you can look at a mixture of nearly equal parts cornstarch and water. If you stir it gently, the mixture flows like a liquid. However, when struck, its surface suddenly hardens. You can mold it into a ball, but once pressure is released, the ball disintegrates.
Here’s how it works: the fluid is a colloid, composed of minuscule particles suspended in liquid. The particles gently repel one another, allowing them to move freely without clumping or sinking to the bottom. But when a forceful impact occurs, the repulsive forces between the particles are overcome, and they coalesce into masses called hydroclusters. Once the energy dissipates, the particles once again repel each other, the hydroclusters disband, and the material returns to its liquid form.
Before an impact, the particles in shear-thickening fluid exist in a balanced state. After an impact, they aggregate, forming solid-like structures.The fluid used in body armor consists of silica particles suspended in polyethylene glycol. Silica, a component of sand and quartz, is combined with polyethylene glycol, a polymer often used in laxatives and lubricants. The silica particles are only a few nanometers across, making this fluid an example of nanotechnology in many reports.
To create liquid body armor with shear-thickening fluid (STF), researchers first dilute the fluid in ethanol. The Kevlar is then soaked in this diluted solution and placed in an oven to evaporate the ethanol. The STF permeates the Kevlar, where the strands help hold the particle-filled fluid in place. Upon impact, the fluid instantly solidifies, enhancing the strength of the Kevlar. This process occurs within milliseconds, after which the armor becomes flexible again.
In laboratory tests, STF-treated Kevlar maintains the same flexibility as untreated Kevlar. The key difference is its increased strength, meaning armor made from STF-treated Kevlar requires fewer layers to provide equivalent protection. For instance, four layers of STF-treated Kevlar absorb the same amount of energy as 14 layers of plain Kevlar. Additionally, STF-treated fibers don't stretch as much upon impact, reducing the penetration of bullets and minimizing damage to underlying tissue. Researchers believe this is due to the greater energy required to stretch the STF-treated fibers.
Photo illustrating treated Kevlar after bullet impact
Photo Courtesy of U.S. Army/Photographer Sgt.Lorie JewellOngoing research into STF-based liquid body armor is being conducted at the U.S. Army Research Laboratory and the University of Delaware. Meanwhile, at MIT, researchers are exploring a different fluid for potential use in body armor. Their work will be explored in the following section.
Magnetorheological Fluid
Magnetorheological fluid becomes organized along the lines of a magnetic field, with its particles aligning in response to the field.Another fluid that enhances Kevlar armor is magnetorheological (MR) fluid. MR fluids are oils filled with iron particles, which are typically coated with surfactants to keep them suspended in the fluid. These iron particles usually make up 20 to 40 percent of the fluid's total volume.
The iron particles in MR fluid are very small, ranging from 3 to 10 microns, but they have a significant impact on the fluid's behavior. When exposed to a magnetic field, these particles align, causing the fluid to thicken dramatically. The term 'magnetorheological' refers to this phenomenon. Rheology, the study of how materials change under force, explains how the magnetic field influences both the shape and viscosity of MR fluids.
The hardening process occurs in approximately twenty thousandths of a second. Its effectiveness can vary significantly based on the fluid's composition and the strength, shape, and size of the magnetic field. For instance, MIT researchers initially used spherical iron particles, which tend to slip past one another even when exposed to a magnetic field. This limits the armor's hardness, prompting the researchers to investigate other particle shapes that might offer greater effectiveness.
Just like shear-thickening fluids, MR fluids can be observed with common household items. A mixture of iron filings and oil offers a clear example. When there is no magnetic field, the fluid moves easily, but when a magnet is introduced, the fluid becomes thicker or even changes its shape, forming distinct peaks, troughs, and other shapes. This dramatic transformation has even been used by artists to create magnetic art using magnets and MR fluids or similar ferrofluids.
With the right combination of particle shape, density, and magnetic field strength, MR fluid can transition from a liquid to a highly viscous solid. Much like shear-thickening fluid, this change has the potential to significantly enhance the strength of armor. The challenge lies in activating this change, as magnets large enough to influence an entire suit would be unwieldy. Therefore, researchers suggest embedding small circuits within the armor to trigger the fluid’s transformation.
Magnetorheological fluid before and after exposure to a magnetic fieldWhen no current flows through the wires, the armor remains soft and flexible. However, as soon as the switch is flipped, electricity flows through the circuits, generating a magnetic field that causes the armor to stiffen and harden instantly. Turning the switch off halts the current, and the armor returns to its flexible state.
In addition to enhancing armor to be stronger, lighter, and more flexible, fabrics infused with shear-thickening and magnetorheological fluids offer additional potential uses. For example, such treated materials could be utilized to create bomb blankets that are easily foldable and portable while still providing protection from explosions and flying debris. Jump boots treated with this technology could become rigid upon impact or activation, ensuring safety for paratroopers. Prison guards' uniforms could also benefit from liquid armor technology, especially given that blunt force weapons and homemade blades are the primary threats they face.
That said, these technologies come with a set of advantages and disadvantages. Here's a quick overview:
