Dielectrowetting: Controlling Oleo- and Hydrophilicity and Shaping Liquid Surfaces

液体如何润湿固体表面对广泛的科学学科和技术应用至关重要，从在半导体晶片上创建薄膜，到表面的粘合和涂层，到在DNA微阵列上有效地沉积和混合液滴。静电场可以改变液体润湿固体表面的效率。近年来，均匀电场被用来控制和操纵导电(含离子)液体的液滴，通常是盐溶液，方法是将液-固接触区域作为电容结构中的一个电极--即所谓的电润湿。这导致了新的电压控制变焦液体透镜、基于液体的电子纸和基于液滴的微流控系统。电润湿的关键是外加电压能够可逆地增加固体表面的有效亲水性，并在不改变表面化学的情况下减少液滴的接触角。然而，许多感兴趣的液体不导电，需要三明治式的电容结构和与液体的直接物理接触限制了它的应用范围。在这个项目中，我们创造了一种新的方法，通过利用液体的介电性质来控制材料的亲水性和亲油性，但影响局限在界面上。与专注于离子的电润湿不同，我们的方法侧重于液体中的偶极。使用非均匀电场会在偶极子的两端产生不相等的力。由此产生的介电泳力可以导致液体移动和重新分布到最高场梯度的区域。我们项目的基础是理解当液体具有固-液、液-气或液-液界面时，介电能量的变化可以耦合到表面自由能的变化。在适当的衰减电场作用下，液体介电可被限制在固-液界面或液-气(或液-液)界面，并可用于非导电液体。通过使用微型交指电极，可以在固体表面上产生衰减的、因此不均匀的电场。对于厚度大于电场衰减长度的液滴，补偿液体介电能量变化的表面能的主要变化是通过改变与固体的接触面积，因此这可以是一种可逆地控制接触角从而控制表面亲水性和亲油性的方法。对于薄液膜，补偿液体介电能量变化的表面能的主要变化是通过改变液-气(或液-液)界面的形状，因此，在这种情况下，它成为一种形成液体表面的方法。在这种将液体介电的影响局部化到界面的方法中，与电润湿不同的是，1.电场是不均匀的；2.电场是由表面微制造的共面电极结构而不是三明治电极结构产生的；3.作用力作用于液体中的偶极子，因此可以是非导电的(或导电的)，而不是导电液体的离子；4.该方法不会受到接触角饱和的影响，因此可以用来产生液膜。该项目的研究旨在建立一种润湿方法，允许使用电场以补充电润湿的方式操纵导电和非导电液体。该项目将提供必要的理解，使新型液滴微流体、液体微驱动、基于液体的光学和显示器的未来发展成为可能。该项目包括在微流控液体处理、芯片实验室设备、显示设备和光流控系统的开发和商业化方面拥有专业知识的工业合作伙伴。

How liquids wet solid surfaces is of fundamental importance to a wide-range of scientific disciplines and technological applications from creating thin films on semiconductor wafers, through adhesion and coating of surfaces, to effective droplet deposition and mixing on DNA microarrays. Electrostatic fields can alter how effectively a liquid wets a solid surface. In recent years uniform electric fields have been used to control and manipulate droplets of conducting (ion containing) liquids, typically a salt solution, by using the liquid-solid contact area as one electrode in a capacitive structure - so called electrowetting. This has led to new voltage controlled variable focus liquid lenses, liquid-based electronic paper and droplet-based microfluidic systems. The key to electrowetting is the ability of an applied voltage to reversibly increase the effective hydrophilicity of a solid surface and reduce the contact angle of the droplet without altering the surface chemistry. However, many liquids of interest are not conducting and the need for a sandwich-style capacitive structure and direct physical contact to the liquid limits its range of applicability.In this project we create a new method of controlling hydrophilicity and oleophilicity of materials by using the dielectric properties of liquids, but with the effects localized to an interface. Unllike electrowetting which focuses on the ions, our method focuses on the dipoles in a liquid. Using a non-uniform electric field generates unequal forces on the two ends of the dipole. The resulting dielectrophoretic force can result in movement and redistribution of the liquid into the areas of highest field gradient. The basis of our project is the understanding that when the liquid has solid-liquid, liquid-vapor or liquid-liquid interfaces, dielectric energy changes can be coupled to surface free energy changes. With a suitable decaying electric field, the effects of liquid dielectrophoresis can be confined to either the solid-liquid interface or to the liquid-vapor (or liquid-liquid) interface and can be used with a non-conducting liquid.By using microfabricated interdigitated electrodes a decaying, and hence non uniform, electric field can be created above a solid surface. For a droplet thicker than the decay length of the electric field, the major change of the surface energy compensating liquid dielectrophoretic energy changes is via a change in the contact area with a solid and so this can be a method of reversibly controlling the contact angle and, hence, the hydro- and oleo- philicity of a surface. For a thin liquid film the major change of the surface energy compensating liquid dielectrophoretic energy changes is via a change in the shape of the liquid-vapor (or liquid-liquid) interface and so, in this case, it becomes a method for shaping a liquid surface. In this method of localizing the effects of liquid dielectrophoresis to an interface the contrast to electrowetting is that,1. the electric fields are non-uniform;2. the electric fields are generated by surface microfabricated co-planar rather than sandwich electrode structures;3. the forces act upon the dipoles in the liquids, which can therefore be non-conducting (or conducting), rather than upon ions of conducting liquids;4. the method does not suffer from saturation of the contact angle and so can be used to produce liquid films. The research in this project seeks to establish an approach to wetting that allows conducting and non-conducting liquids to be manipulated using electric fields in a manner complementary to electrowetting. The project will provide the understanding needed to allow future development of novel droplet microfluidic, liquid microactuation, liquid-based optics and displays. The project includes industrial partners who have expertise in the development and commercialisation of microfluidic liquid handling, lab-on-chip devices, display devices and optofluidic systems.