Using the emulsion glass transition to test the universality of jamming

许多普通材料都是柔软的固体。 实例包括胶体糊剂（如牙膏）、凝胶（如明胶）、泡沫（如剃须膏）和乳液（如蛋黄酱）。 这些材料很柔软：例如，牙膏不费多大力气就能从牙膏管中挤出来。 然而，它们是堵塞的固体：你可以让一堆牙膏，而不是让它变成一滩。 近二十年来，科学家们已经证实，这些软材料的固体性质类似于窗户玻璃的固体性质。 最近，计算机模拟表明，软材料的固体性质与玻璃的固体性质有根本的不同。 关键的区别被认为是由于温度引起的振动运动，这对玻璃中的分子很重要，对软材料的成分不重要。 该项目使用乳液（油滴在肥皂水）来解决这些问题。 一些样品是用非常小的液滴制成的，温度引起的振动对这些液滴至关重要;而另一些样品是用大液滴制成的，温度引起的振动可以忽略不计。 目标是在所有其他细节保持不变的情况下直接比较两种类型样本的行为，从而直接测试模拟预测。 技术摘要PI和他的实验室小组使用浓缩乳剂作为干扰转换普遍性的实验测试。 乳液是不混溶液体中的液滴，其中添加了表面活性剂以防止液滴聚结。 在低液滴浓度下，乳液容易流动。 在更高的液滴浓度下，液滴彼此接触并且样品变得“堵塞”。“作为一种堵塞的材料，它在微观上保持无序，但在宏观上具有屈服应力，看起来像固体，尽管是一种相当软的固体。 在过去的20年里，有许多说法认为，这种在乳液（或其他软材料）中向无定形固体的转变与常规的玻璃化转变相同。 然而，最近的模拟表明，玻璃化转变和堵塞转变是不同的，前者发生在热材料和后者的非热材料。PI和他的实验室小组使用乳剂通过改变液滴大小从热极限调整到非热极限。流变学用于测量样品中的应力和应变速率之间的关系。研究了流变行为随液滴浓度的变化，获得了直接测试玻璃化转变和堵塞转变是否明显的数据。此外，显微镜被用来查看剪切样品，以显示什么微观行为引起的流变行为。实验室小组还为小学生举办实地考察，让来访的学生亲身体验实验室中使用的粘稠材料，如实验中使用的稠密乳液。 这些材料挑战了材料只能以固体、液体或气体形式出现的想法。

Non-technical AbstractMany common materials are soft jammed solids. Examples include colloidal pastes (such as toothpaste), gels (such as gelatin), foams (such as shaving cream), and emulsions (such as mayonnaise). These materials are soft: for example, toothpaste squirts out of the tube without much effort. However, they are jammed solids: you can make a pile of toothpaste, rather than having it turn into a puddle. For nearly two decades, scientists have conjectured that the solid-like properties of these soft materials are similar to the solid-like properties of window glass. Very recently, computer simulations suggested the solid-like properties of soft materials are fundamentally different than the solid-like properties of a glass. The key difference is thought to be the vibrational motions due to temperature, which are important for molecules in a glass and unimportant for the constituents of soft materials. This project uses emulsions (oil droplets in soapy water) to address these questions. Some samples are made using very tiny droplets, for which temperature-induced vibrations are crucial; and other samples are made using large droplets, for which temperature-induced vibrations are negligible. The goal is to directly compare behavior of the two types of samples with all other details held constant, thus directly testing the simulation predictions. Technical AbstractThe PI and his lab group use concentrated emulsions as an experimental test of the universality of the jamming transition. Emulsions are liquid droplets in an immiscible liquid, where a surfactant has been added to prevent the droplets from coalescing. At low droplet concentration, emulsions flow easily. At higher droplet concentration, the droplets contact one another and the sample becomes "jammed." As a jammed material, it remains microscopically disordered but macroscopically has a yield stress and appears solid-like, albeit a quite soft solid. Over the past 20 years there have been many statements that this transition to an amorphous solid in emulsions (or other soft materials) is the same as the regular glass transition. However, recent simulations suggest that the glass transition and jamming transition are distinct, with the former occurring for thermal materials and the latter for athermal materials. The PI and his lab group use emulsions to tune from the thermal to athermal limit by varying the droplet sizes. Rheology is used to measure the relationship between stress and strain rate in the samples. The change of rheological behavior is investigated as the droplet concentration is varied, obtaining data directly testing if the glass and jamming transitions are distinct. Additionally, microscopy is used to view sheared samples to show what microscopic behaviors give rise to the rheological behavior. The lab group also hosts field trips for primary school students, giving the visiting students hands-on laboratory experiences with the squishy materials such as the dense emulsions used in the experiments. Such materials challenge ideas that materials only come as solids, liquids, or gases.