Jafari Kang Saeed, Vandadi Vahid, Felske James D, Masoud Hassan
Department of Mechanical Engineering, University of Nevada, Reno, Nevada 89557, USA.
Department of Mechanical and Aerospace Engineering, State University of New York, Buffalo, New York 14260, USA.
Phys Rev E. 2016 Dec;94(6-1):063104. doi: 10.1103/PhysRevE.94.063104. Epub 2016 Dec 12.
When a colloidal sessile droplet dries on a substrate, the particles suspended in it usually deposit in a ringlike pattern. This phenomenon is commonly referred to as the "coffee-ring" effect. One paradigm for why this occurs is as a consequence of the solutes being transported towards the pinned contact line by the flow inside the drop, which is induced by surface evaporation. From this perspective, the role of the liquid-gas interface in shaping the deposition pattern is somewhat minimized. Here, we propose an alternative mechanism for the coffee-ring deposition. It is based on the bulk flow within the drop transporting particles to the interface where they are captured by the receding free surface and subsequently transported along the interface until they are deposited near the contact line. That the interface captures the solutes as the evaporation proceeds is supported by a Lagrangian tracing of particles advected by the flow field within the droplet. We model the interfacial adsorption and transport of particles as a one-dimensional advection-generation process in toroidal coordinates and show that the theory reproduces ring-shaped depositions. Using this model, deposition patterns on both hydrophilic and hydrophobic surfaces are examined in which the evaporation is modeled as being either diffusive or uniform over the surface.
当一个胶体静态液滴在基底上干燥时，悬浮在其中的颗粒通常会以环状图案沉积。这种现象通常被称为“咖啡环”效应。关于这种现象发生的一种范例解释是，由于表面蒸发引起液滴内部的流动，溶质被输运到固定的接触线处。从这个角度来看，液 - 气界面在塑造沉积图案方面的作用在某种程度上被最小化了。在这里，我们提出了一种关于咖啡环沉积的替代机制。它基于液滴内部的整体流动将颗粒输运到界面，在那里它们被后退的自由表面捕获，随后沿着界面输运，直到它们在接触线附近沉积。随着蒸发的进行，界面捕获溶质这一现象得到了通过拉格朗日追踪液滴内流场平流的颗粒的支持。我们将颗粒的界面吸附和输运建模为环形坐标中的一维平流 - 生成过程，并表明该理论能够重现环形沉积。使用这个模型，研究了亲水性和疏水性表面上的沉积图案，其中蒸发被建模为在表面上是扩散的或均匀的。
