芡糊是液體還是固體？

Oobleck的剪切增稠：微小顆粒間的摩擦造就了玉米澱粉溶液的神奇性質。

撰文 / 柯林斯（Nathan Collins）  翻譯 / 洪艾彊
芡糊這種濃稠的溶液（在太白粉或玉米澱粉裡加進些許的水）有著怪異的性質。乍看之下，它和一般的液體沒有兩樣，可以倒進不同形狀的容器中，或把手插 進去。但是如果突然擠壓或是用力拍擊液面，你會發現它突然變硬了──因此你也可以把它揉成球狀，或是在它的表面行走，甚至當成彈簧床在上面跳來跳去。

用力攪拌芡糊會使它變得像石頭一樣堅硬。美國耶魯大學的布朗（Eric Brown）特別喜歡把玉米澱粉均勻混入水中，示範這種稱為「毆不裂」（oobleck）物質的奇特物性。他說，只要夠用力，毆不裂可以使攪拌用的金屬棒 攔腰折斷，更神奇的是，一旦放慢攪拌的速度，又會回復為正常的液體。這兩種截然不同的軟硬反應可以不斷重複出現，直到你手痠為止。

物理學家嘗試多年，想解開剪切增稠（shear thickening）現象的原理，這種現象是指有些液體能突然轉變為固體。直到2003年，法國科學家終於找到了一些證據，顯示顆粒間的摩擦可能是造成剪切增稠現象的主因。

隨後，其他科學家利用電腦模擬證實了顆粒間的作用力確實是關鍵。最近在《物理評論通訊》有一篇關於此現象的文章，美國紐約市立大學的化工系教授莫瑞 斯（Jeffrey Morris）是共同作者之一，他說：「在玉米澱粉含量低的時候，溶劑扮演潤滑的角色，使顆粒（即溶質）得以自由移動。」即使再多加點玉米澱粉，他補充 說：「水所扮演的潤滑角色依然不變，」直到有人突然用力去攪拌它。該研究的主要作者、紐約市立大學的瀨戶良平表示，外力迫使顆粒產生碰撞，而粗糙的顆粒表 面使得它們不容易打滑，逐漸形成一條條堅硬的長鏈，這就是剪切增稠使玉米澱粉溶液突然變硬的原因。

莫瑞斯說：「剪切增稠實在太神奇了。」想想看，這麼簡單的現象竟然花費研究者無數次的實驗和理論分析，才得以稍微窺見端倪。布朗認為還有更多的疑問有待解答，例如，同樣的微觀作用力是否也能解釋毆不裂物質受到拍擊時所表現的異常硬度。刊載於《科學人》2014年第145期3月號
SA原文：Friction Makes Cornstarch and Water into Bizarre “Oobleck"

Friction Makes Cornstarch and Water into Bizarre "Oobleck"

 Friction between tiny particles explains the bizarre properties of cornstarch in fluid

Cornstarch mixed with a little water is peculiar stuff. At first glance it seems like any other liquid—you can pour it from one bowl to another or dip your hands in it. But give it a squeeze or strike the surface of the fluid with a hard blow, and the cornstarch slurry suddenly firms up—you can roll it into balls, walk on it and even bounce on it.

Vigorously stirring the mixture will also turn it nearly to stone. Yale University physicist Eric Brown is fond of demonstrating the weirdness of cornstarch and water, sometimes called Oobleck, by mixing them together with a metal shaft. Stir forcefully enough, he says, and he can actually break the rod. Stranger still, the transition is reversible: ease up on the stirring, and what seemed solid turns right back to liquid.

Physicists long struggled to fully account for the rapid liquid-to-solid shift, known as shear thickening. Eventually, in 2003, a team of French experimenters saw the first hints that shear thickening is the by-product of friction between the particles.

More recently, researchers have confirmed that view with detailed simulations of particle interactions. At low starch concentrations, the liquid lubricates the particles, allowing them to move more or less freely, says co-author Jeffrey Morris, a professor of chemical engineering at the City University of New York who co-authored a new study on the phenomenon in Physical Review Letters. Even with more particles, water still “has that nearly perfect lubricating role,” Morris says, until someone starts stirring a little too hard. The extra force slams suspended particles together, and their rough surfaces prevent particles from sliding past one another. Instead they form long, rigid chains held together by friction, which give shear-thickened fluids their near-solid feel, says lead study author Ryohei Seto, also at C.U.N.Y.

“Shear thickening is remarkable,” Morris says, noting that it took countless experiments and theoretical studies to answer “a basic question” in physics. Many more questions remain, Brown says. It is not yet clear, for instance, whether the same microscopic interactions responsible for shear thickening also account for Oobleck's impact resistance.

This article was originally published with the title "Instant Weirdness—Just Add Water."