Accelerated versus decelerated settling velocity of a drop

1236316 ward乳剂是在重力作用下沉降的液-液混合物。与生物应用相关或来自生物材料的稀分散相乳剂的例子包括药物（如异丙酚）、局部乳膏（如面霜）和植物油燃料乳剂。这些乳剂都来源于某种形式的脂质（脂肪酸、甘油三酯），这些脂质可以通过水解或皂化化学反应直接转化为表面活性剂。将一定比例的脂肪酸直接转化为表面活性剂将为稳定某些生物相关乳剂提供一个简单的一步过程。众所周知，当表面活性剂从本体分散相吸附到界面时，将导致液滴的减速运动。减速运动是由浓度梯度产生的马兰戈尼应力引起的，这是由于表面活性剂在液滴后部积聚而反对液滴沉降。研究人员将研究一种产生解吸主导运动的过程，这种运动将导致马兰戈尼应力的逆转，因此与具有清洁界面的液滴沉积相比，会产生加速的液滴运动。为了产生引起液滴加速的应力，必须在界面上产生比长时间经过的平衡表面覆盖值更多的表面活性剂。研究人员将证明，这种应力逆转可能是通过在界面上通过快速化学反应产生表面活性剂来产生的。本文的主要目的是研究粘性液体中加速和减速的稳定低雷诺数液滴沉降。这个项目的独特之处在于，在界面化学反应产生的稀表面活性剂存在的情况下，液滴沉降。这项研究包括实验和数学/计算分析的结合。目的是通过实验测量单个液滴的平移速度作为输入参数的函数，这些参数是反应物类型（植物油和分散的氢氧化钠水溶液）、反应物浓度和液滴体积。在实验的同时，利用Legendre多项式和Gegenbauer多项式的幂级数展开进行了数学分析，以满足包含吸附/解吸热力学关系和化学反应动力学以及Stokes流动极限动量输运方程的稳定界面物质守恒定律。该项目将探索与单个液滴沉降速度相关的新物理现象。这项研究应该为当前和未来利用稀释乳剂长期储存的工艺提供额外的见解，因为这些工艺取决于单个沉淀液滴的速度。进一步了解长期储存过程对于由生物材料生产的乳剂至关重要，因为分解限制了它们的保质期。一名来自HBCU的本科生将在夏季期间在PI的实验室工作以参与该项目。研究结果亦会透过部门网站及在会议上公布。

1236316WardEmulsions are liquid-liquid mixtures that are subject to sedimentation driven by gravitational forces. Examples of dilute-dispersed phase emulsions that are relevant for biological applications, or derived from bio-materials, include pharmaceutical drugs (e.g. propofol), topical creams (e.g. facial creams) and vegetable oil fuel emulsions. These emulsions are all derived from some form of lipid (fatty acid, triglyceride) that may be converted directly in a surfactant via a hydrolysis or saponification chemical reaction. Converting a percentage of the fatty acids directly into a surfactant would provide an easy one step process for stabilizing certain biologically relevant emulsion. It is well known that surfactants, when adsorbing to an interface from the bulk-dispersed phase, will lead to decelerated motion of a droplet. The decelerated motion is caused by a Marangoni stress generated by concentration gradients due to the accumulation of surfactant at the rear of the droplet that oppose droplet sedimentation. The researchers will study a process to generate desorption dominated motion which would cause a reversal in the Marangoni stresses and therefore produce accelerated droplet motion when compared with sedimentation of a droplet with a clean interface. To generate stresses that cause droplet acceleration one must produce more surfactant at the interface than the long elapsed time, equilibrium surface coverage value. The researchers will demonstrate that this stress reversal may be produced by generating surfactant at the interface through a fast chemical reaction. A key objective of this proposal is a study of both accelerated and decelerated steady low Reynolds number droplet sedimentation in a viscous liquid. The unique feature in this project is droplet sedimentation in the presence of dilute surfactant species generated by an interfacial chemical reaction. The study includes a combination of experiments and mathematical/computational analyses. The goal is to experimentally measure the translational velocity of a single droplet as a function of the input parameters which are the reactant types (vegetable oils and dispersed-aqueous sodium hydroxide), concentration of reactants, and droplet volume. Parallel to the experiments, a mathematical analysis is performed using a power series expansion of Legendre and Gegenbauer polynomials to satisfy the steady interfacial species conservation laws containing thermodynamic relationships for adsorption/desorption and chemical reaction kinetics combined with momentum transport equations in the Stokes flow limit.This project will explore novel physical phenomenon associated with the sedimentation speed of a single droplet. The research should provide additional insight for current and future processes that utilize the long term storage of dilute emulsions since these processes depend on the velocity of a single sedimenting droplet. Further understanding of long term storage processes will be vital for emulsions produced from biological material since decomposition limits their shelf life. An undergraduate student from an HBCU will work in the PI's lab during the summer to participate in the project. The research results also will be disseminated though departmental websites and at conference meetings.