Introduction Materials and Methods Hypothesis Results Enzymatic Modification of Amylomaize Grains to Enhance Shear-Thickening Ability Stephen Hill -- Colonia High School Introduction Materials and Methods A shear-thickening fluid is any liquid whose apparent viscosity decreases with duration of shear stress (Cheremisihoff, 1988). Shear-thickening fluids (STFs) are a type of non-Newtonian fluid suspension that momentarily exhibit the properties of a solid when force is applied quickly (Barnes, 1989). The apparent increase in viscosity of these colloidal suspensions is reversible and results from the aggregation of suspended particles during the application of stress (Bossis and Brady, 1989). This thickening property is currently being researched by the United States Army for use in body armor as a lightweight, flexible protectant. Lee, et. al., (2004) showed that there was a decrease in the depth at which projectiles penetrated Kevlar when the fabric had been impregnated with a silica-based STF. Currently overlooked as a source of shear thickening material for use in body armor, the ability of corn starch suspensions to thicken dramatically upon impact are frequently demonstrated in science textbooks (Danch, 2009) and popular science demonstrations and internet videos (www.youtube.com). Previous research (Hill, 2009), tested the effectiveness of an amylomaize (corn starch) based STF. It was concluded that an amylomaize based STF layer of 3.0 cm was effective at stopping low velocity projectiles ( ~ 10m/s) as measured by their depth of penetration into a clay target . Amylomaize based STFs may be a low-cost alternative to the engineered silica nanoparticle fluids currently being researched. The effectiveness of an engineered silica nanoparticle STF in ballistic protection was directly related to the surface configuration of the particle. Silica nanoparticles with a more rough, porous and jagged surface showed the greatest shear-thickening abilities (Wetzel et. al., 2004). Amylomaize grains by comparison, are round and relatively smooth in nature and yet exhibit shear-thickening properties (fig. 1). Step 1 Step 2 Step 3 Step 4 Fig. 1. Microscopic image detailing the relatively smooth surface of cornstarch grains (400X). Figure 3. Diagram of the “masking and etching” process of modifying corn starch grains Figure 2. Microscopic image of a rough porous silica nano-particle Hypothesis If amylomaize particles were engineered to resemble the rough and jagged surfaces seen in the most effective engineered silica particles, a suspension containing these modified amylomaize grains would exhibit increased shear-thickening properties. Figure 4. Image of fully functioning rheometer as it measures shear-thickening property of suspension. Results Figure 5. Schematic drawing of rheometer Figure 6. Image of shear-thickening fluid as it is being spun on the rheometer. Before After Figure 7. Shear-thickening property of sodium silicate/amylase treated starch compared to untreated starch as measured by cylinder spin rate. Figure 9. Shear-thickening property of amylase treated starch compared to untreated starch as measured by cylinder spin rate. Figure 8. Before and after microscopic images of starch grains exposed only to amylase for 36 hours. Figure 10 . 400x microscopic image of silicate coated corn starch particles. Discussion and Conclusion Applications As hypothesized, starch grains engineered with a silicate mask and enzyme etching showed a significant increase in shear-thickening abilities as measured by a rheometer. The first test shows that by treating starch grains only with amylase, shear-thickening ability is decreased when the grains are suspended in a liquid. The second test shows that coating the starch grains with sodium silicate slightly increases the shear thickening property when the grains are suspended in a liquid. The third test shows that a synergy of the two treatments displays large increase in shear-thickening abilities when the grains are suspended in a liquid. The effectiveness of the modified starch particles described in this experiment may present another option in the use of shear-thickening fluids: Body armor. It should be noted that corn starch is an inexpensive and readily available source of material . The treatment process developed during this research is also inexpensive and relatively simple. Figure 11. Shear-thickening property of sodium silicate treated starch compared to untreated starch as measured by cylinder spin rate. Figure 12. The shear-thickening ability of amylase and silicate treated starch suspensions compared to the other treatments