Accelerated versus decelerated settling velocity of a drop

1236316Ward乳状液是一种液体-液体混合物，在重力的驱动下会发生沉淀。与生物应用相关或源自生物材料的稀散相乳剂的例子包括药物(例如异丙酚)、外用乳膏(例如面霜)和植物油燃料乳剂。这些乳状液都是从某种形式的脂肪(脂肪酸、甘油三酯)中提取出来的，可以通过水解或皂化化学反应在表面活性剂中直接转化。将一定比例的脂肪酸直接转化为表面活性剂将提供一个简单的一步过程来稳定某些生物相关的乳状液。众所周知，当表面活性剂从本体分散相吸附到界面时，会导致液滴减速运动。减速运动是由Marangoni应力引起的，该应力是由浓度梯度引起的，该浓度梯度是由于表面活性物质在液滴后面积累而与液滴沉积相反。研究人员将研究一种产生解吸主导运动的过程，与界面干净的液滴的沉降相比，这将导致Marangoni应力的逆转，从而产生加速的液滴运动。为了产生导致液滴加速的应力，必须在界面产生比经过较长时间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快速化学反应在界面上产生表面活性物质而产生的。这一方案的一个关键目标是研究加速和减速稳态低雷诺数液滴在粘性液体中的沉积。该项目的独特之处在于，在界面化学反应产生的稀释型表面活性物质存在的情况下，液滴会发生沉淀。这项研究包括实验和数学/计算分析相结合。其目标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，这些参数包括反应物类型(植物油和分散的氢氧化钠)、反应物浓度和液滴体积。在实验的同时，利用Legendre和Gegenbauer多项式的级数展开式进行数学分析，以满足稳定的界面物种守恒定律，包括吸附/脱附和化学反应动力学的热力学关系，结合Stokes流动极限下的动量输运方程。本项目将探索与单个液滴的沉积速度相关的新的物理现象。这项研究应该为目前和未来利用稀乳剂长期储存的过程提供更多的洞察力，因为这些过程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。暑假期间，HBCU的一名本科生将在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.