Accelerated versus decelerated settling velocity of a drop

1236316 WardEmulsions是液-液混合物，在重力作用下发生沉降。与生物学应用相关或衍生自生物材料的稀释分散相乳液的实例包括药物（例如丙泊酚）、局部乳膏（例如面霜）和植物油燃料乳液。这些乳剂均来源于某种形式的脂质（脂肪酸、甘油三酯），其可通过水解或皂化化学反应直接转化为表面活性剂。将一定百分比的脂肪酸直接转化为表面活性剂将提供用于稳定某些生物相关乳液的简单的一步法。 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，浓度梯度是由于表面活性剂在液滴后部的积累而产生的，表面活性剂与液滴沉降相反。研究人员将研究一个过程，以产生解吸为主的运动，这将导致在马兰戈尼应力逆转，因此产生加速液滴运动相比，沉降液滴与一个干净的界面。为了产生导致液滴加速的应力，必须在界面处产生比长时间经过的平衡表面覆盖率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快速化学反应在界面处产生表面活性剂而产生的。该建议的一个关键目标是研究粘性液体中的加速和减速稳态低雷诺数液滴沉降。该项目的独特之处是在界面化学反应产生的稀表面活性剂存在下的液滴沉降。这项研究包括实验和数学/计算分析的结合。我们的目标是实验测量的平移速度的一个单一的液滴作为输入参数的函数，这是反应物的类型（植物油和分散的氢氧化钠水溶液），反应物的浓度，和液滴体积。在实验的同时，利用Legendre和Gegenbauer多项式的幂级数展开进行数学分析，以满足稳态界面物种守恒定律，该定律包含吸附/脱附和化学反应动力学的热力学关系，并结合Stokes流动极限下的动量输运方程。 这项研究应该提供额外的见解，目前和未来的过程中，利用长期储存的稀乳液，因为这些过程取决于一个单一的沉降液滴的速度。进一步了解长期储存过程将是至关重要的乳液生产的生物材料，因为分解限制其保质期。一名来自HBCU的本科生将在暑期在PI的实验室工作，参与该项目。研究结果还将通过部门网站和会议进行传播。

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.