Self-Propelled Droplet Motion on Gradient Slippery Liquid Infused Porous Surfaces (G-SLIPS)

控制液滴如何与表面相互作用的能力是一个广泛的科学兴趣话题，并有许多实际应用。从建筑外立面或家用电器的自清洁表面，一直到医疗应用和自我诊断，人们对能够完成无数任务的智能材料的需求越来越大，而且非常宝贵。最近，人们对仿生系统越来越感兴趣，例如基于荷叶效应的超疏水表面，以及最近基于猪笼草的光滑液体注入多孔表面（SLIPS）。这类表面由于其在涂层和喷墨打印等应用中的防水和防结冰性能以及控制润湿的能力而备受关注。当处理微观尺度的液体时，液滴的表面积与体积之比变得越来越大，在运动过程中必须克服摩擦力。为了使液滴在表面上运动，还必须克服由于接触角迟滞而产生的静摩擦力。当无柄液滴位于slip上时，其底部周围会出现湿润的脊状结构。这种湿润脊是由于液滴三相接触线上的力的平衡。通常画为诺伊曼三角形，这个脊的形状取决于所涉及的液体的性质。2015年，我与同事（Langmuir, 31(43), 2015）证明了表面上液滴周围湿润脊的高度也取决于下表面的性质和纹理。我们用这样一个表面来表明，我们可以测量一个无底液滴的表观接触角，就像固体表面上的液滴一样，并表明这条表观接触线是高度可移动的，而且滞后很小。由于SLIP表面的润滑性质，放置在这种表面上的液滴可以很容易地移动或被移动。随后，我发展了一个新的想法，如果在SLIP表面的底层地形中引入一个梯度，就有可能在液滴中产生自主运动。该项目将创造低接触角迟滞、低摩擦的表面，并且能够在不需要任何泵送机构的情况下将运动传递给液滴。我将使用下表面纹理的粗糙度梯度，以及注入的液体润滑剂来创建梯度光滑液体注入多孔表面（G-SLIPS）。我将使用滑移表面的这种变化和初步实验的结果，对这种表面上的运动机制进行全面研究。我将通过实验研究诸如润滑液体类型或底层表面地形等参数如何产生和控制液滴运动。最后，我将创建和测试能够与特定微流体任务相称的规定运动的特定表面。该项目最终将为无泵微流体系统的未来发展增加新技术，并创造出既能克服接触线钉住又能使用SLIP surfaces减少粘性摩擦的表面。

The ability to control how liquid droplets interact with surfaces is a topic of wide scientific interests and has many practical applications. From self-cleaning surfaces for building facades or domestic appliances, right through to medical applications and self-diagnostics, there is an increasing and invaluable call for smart materials capable of a myriad of tasks. Recently there has been an increasing interest in biomimetic systems such as super hydrophobic surfaces based on the Lotus leaf effect, and more recently on Slippery Liquid Infused Porous Surfaces (SLIPS) based on the nepenthes pitcher plant. Such surfaces have been the subject of much interest in terms of their capabilities for water shedding and anti-icing properties as well as their ability to control wetting in applications such as coating and inkjet printing.When dealing with liquids on a microscale, the ratio of a droplet's surface area to volume becomes ever larger and frictional forces must be overcome during motion. To induce motion of droplets on a surface, static frictional forces, due to contact angle hysteresis, must also be overcome. When a sessile droplet sits on a SLIPS it exhibits a wetting ridge around its base. This wetting ridge is due to the balance of forces at the three phase contact line of the droplet. Usually drawn as a Neumman Triangle, the shape of this ridge is dependent on the properties of the liquids involved. In 2015, with co-workers, (Langmuir, 31(43), 2015) I showed that the height of the wetting ridge around a droplet on a surface is also dependent on the nature and texture of the underlying surface. We used such a surface to show that one can measure an apparent contact angle, for a sessile droplet, much like that of a droplet on a solid surface and showed that this apparent contact line is highly mobile with very little hysteresis. A droplet placed on such a surface can move or be moved very easily because of the lubricating nature of SLIP surface. Subsequently I have developed the new idea that, if one introduces a gradient into the underlying topography of the SLIP surface, it is possible to generate autonomous motion in a droplet.This project will create surfaces which give low contact angle hysteresis, low friction and are capable of imparting motion onto droplets without the need for any pumping mechanism. I will use gradients in the roughness of an underlying surface texture, along with an infused liquid lubricant to create a Gradient Slippery Liquid Infused Porous Surface (G-SLIPS). I will use this variation of a SLIP surface and results from preliminary experiments to perform a full study into the mechanisms of motion on such surfaces. I will experimentally examine how parameters such as the type of lubricating liquid or the underlying surface topography create and control droplet motion. Finally, I will create and test specific surfaces capable of prescribed motion commensurate with specific microfluidic tasks. This project will ultimately add new techniques to the future development of pump free microfluidic systems and create surfaces that overcome both contact-line pinning and reduce viscous friction using SLIP Surfaces.