Spreading of viscous fluid drops on a solid substrate assisted by thermal fluctuations

We study the spreading of viscous drops on a solid substrate, taking into account the effects of thermal fluctuations in the fluid momentum. A nonlinear stochastic lubrication equation is derived and studied using numerical simulations and scaling analysis. We show that asymptotically spreading drops admit self-similar shapes, whose average radii can increase at rates much faster than these predicted by Tanner's law. We discuss the physical realizability of our results for thin molecular and complex fluid films, and predict that such phenomenon can in principal be observed in various flow geometries.     

液滴在梯度微结构表面上的铺展动力学分析

本文通过改变肋柱宽度和间距, 构造了二级和多级梯度微结构表面, 采用格子-Boltzmann方法对液滴在两种梯度表面上的铺展过程进行了研究, 探析液滴运动的机理和调控方法. 结果表明, 在改变肋柱间距的二级梯度表面上, 当液滴处于Cassie态时, 接触角滞后大小与粗糙度梯度成正比关系; 当液滴从Cassie态转换为Wenzel态或介于两者之间的不稳定态时, 这一正比关系不再遵循. 在改变肋柱宽度的二级梯度表面上, 接触角滞后大小与粗糙度梯度始终成正比关系. 在多级梯度表面上, 随液滴初始半径增大, 接触角滞后减小, 但液滴平衡位置相较于初始位置偏离增大. 对梯度微结构表面上液滴运动和接触角滞后的定量分析, 可为实现梯度微结构表面液滴运动调控提供理论依据.     

Three-lobed shape bifurcation of rotating liquid drops

The evolution of axisymmetric equilibrium shapes of a rigidly rotating liquid drop can be extended beyond the 2-lobed shape bifurcation point if the rotating drop is driven in the n = 2 axisymmetric shape oscillation (perturbation), where n is the mode of oscillation. A reason for the extended stability of the perturbed rotating drop is that the inertia of the driven axisymmetric shape oscillation suppresses growth of a natural nonaxisymmetric shape fluctuation which leads to the 2-lobed shape bifurcation. The axisymmetric shape of the drop eventually bifurcates into either a 2- or a 3-lobed shape at a higher bifurcation point which is asserted to be the 3-lobed shape bifurcation point.     

Bouncing behaviors of suspension liquid drops on a superhydrophobic surface

Abstract  

This paper reveals three patterns of bouncing behaviors of suspension drops containing calcium carbonate (CaCO₃) powder on a superhydrophobic surface with the aid of a high-speed camera. In transmission electron microscopy (TEM) observation, the particles of CaCO₃ are shaped like sticks whose equivalent diameters are about 700 nm. Unlike a pure water drop, dense suspension drops cannot be pinched off at the bounce on the superhydrophobic surface due to a high effective viscosity, whereas the equilibrium contact angle appears to be almost identical in all kinds of droplets.     

Features of motion of interacting magnetic liquid drops

The features of simultaneous motion of two identical drops of magnetic liquid, which are determined by their hydrodynamic and magnetic interaction, are considered. The changes in the trajectories of drops initially moving in the same direction, caused by a variation of direction and magnitude of the applied dc and ac magnetic field, are studied experimentally and substantiated theoretically. The possibility of effective control of the motion of magnetic liquid drops by applying magnetic fields is demonstrated.     

Symmetric and asymmetric stabilities of charged liquid drops

The dynamics of a charged drop is described as modes of deformation from the saddle point shapes. On using five deformation parameters, one finds that the symmetric unstable mode tends to increase the thickness of the middle neck, thus forbidding the scission into two equal parts.     

Buckling instability in liquid crystalline physical gels

In a nematic gel we observe a low-energy buckling deformation arising from soft and semisoft elastic modes. We prepare the self-assembled gel by dissolving a coil-side-group liquid-crystalline polymer-coil copolymer in a nematic liquid crystal. The gel has long network strands and a precisely tailored structure, making it ideal for studying nematic rubber elasticity. Under polarized optical microscopy we observe a striped texture that forms when gels uniformly aligned at 35 degrees C are cooled to room temperature. We model the instability using the molecular theory of nematic rubber elasticity, and the theory correctly captures the change in pitch length with sample thickness and polymer concentration. This buckling instability is a clear example of a low-energy deformation that arises in materials where polymer network strains are coupled to the director orientation.     

Cavitation bubble dynamics inside liquid drops in microgravity

We studied spark-generated cavitation bubbles inside water drops produced in microgravity. High-speed visualizations disclosed unique effects of the spherical and nearly isolated liquid volume. In particular, (1) toroidally collapsing bubbles generate two liquid jets escaping from the drop, and the "splash jet" discloses a remarkable broadening. (2) Shock waves induce a strong form of secondary cavitation due to the particular shock wave confinement. This feature offers a novel way to estimate integral shock wave energies in isolated volumes. (3) Bubble lifetimes in drops are shorter than in extended volumes in remarkable agreement with herein derived corrective terms for the Rayleigh-Plesset equation.     

振动诱导液滴与液面不融合现象的研究

发现硅油液滴能够停留在竖直振动的硅油液面上,通过等厚干涉实验说明液滴和液面间存在空气薄膜,分析空气膜形状的变化过程,并依此建立简化模型论述空气膜阻止液滴与液面融合的原理.实验构造了浸没在硅油液面下的硅油液滴,建立理论模型估算空气膜的平均厚度,并与实验估测值对比.本文对多个液滴在液面上呼吸模式的振动现象作了描述和解释.     

Dynamic structure formation at the fronts of volatile liquid drops

We report on instabilities during the spreading of volatile liquids, with emphasis on the novel instability observed when isopropyl alcohol is deposited on a monocrystalline Si wafer. This instability is characterized by emission of drops ahead of the expanding front, with each drop followed by smaller, satellite droplets, forming the structures which we nickname "octopi" due to their appearance. A less volatile liquid, or a substrate of larger heat conductivity, suppresses this instability. We formulate a theoretical model that reproduces the main features of the experiment.     

液态锂限制器的铺展性测试平台设计及初步实验

为研究液态锂在电磁驱动限制器表面的铺展特性,设计了与EAST限制器接口相同的限制器测试平台.该平台运行时真空环境可达10?4Pa,对被测限制器可加热至350℃.在限制器锂回路管道上,利用外部2T水平磁场以及竖直方向施加的最大为200A的直流电流,形成电磁驱动力驱动下的锂液循环回路.测试平台设有顶部和正面两个观察窗,能够...     

Correlated ejection of electron-hole drops from a continuously generated electron-hole liquid in Ge

The photocurrent power spectra were measured in a Ge photodiode as a function of the incident optical intensity at 1.8 K and 4.3 K. The observed spectra showed a peak at low frequencies superimposed on a continuum which has a cut-off at high frequency. The interpretation of the results implies that the electron-hole drops coming from a same region of the photoexcited liquid are ejected periodically.     

Contact angle hysteresis of liquid drops as means to measure adhesive energy of zein on solid substrates

Adhesion of zein to solid substrates has been studied using surface energy profiles as indices and by adhesion mapping using atomic force microscopy (AFM). Different plasticizers like glycerol and sorbitol have been used to form mixed films with zein and properties of these films are studied using surface energy profiles. Comparison of the results from the different mixed samples with those from the pure zein films showed that force mapping could identify areas rich in protein. The adhesion maps produced were deconvoluted from sample topography and contrasted with the data obtained from contact angle measurements. A comparison of the two methods shows that the extent of contact angle hysteresis is indicative of both hydrophobicity of the surface as well as the force of adhesion. Mechanical properties and microstructure of zein films prepared by casting from solutions and using Langmuir-Blodgett film technique have been investigated. Pure zein seemed brittle and exhibited an essentially linear relationship between stress and strain. Films with plasticizer were tougher than these films. In general, mixed films showed better mechanical properties than pure films and had higher ultimate tensile strength and increased per cent elongation. Further, the mixed films of zein showed a higher force of adhesion compared to the pure films.     

不同液膜体系中Marangoni效应对活性剂液滴铺展的影响

针对含可溶性表面活性剂的液滴在预置液膜上的铺展过程,应用润滑理论建立液滴厚度、活性剂表面浓度及内部浓度演化过程的控制方程组,采用PDECOL程序模拟液滴的演化特征,探讨不同液膜体系下Marangoni效应的作用机理,并基于瞬态增长法分析负体系下液滴演化的不稳定性.研究表明,正体系下Marangoni效应可促进活性剂分子扩散,有利于活性剂液滴的稳定铺展;负体系下Marangoni效应则抑制液滴铺展,使液滴在中心区域产生振荡现象,呈现不稳定性特征,且该特征随负效应增强而愈加明显.在正负两种体系下,Marangoni作用力均是促使液滴横向演化的主要驱动力.