Multifunctional TiO2‐Based Particles: The Effect of Fluorination Degree and Liquid Surface Tension on Wetting Behavior

A systematic investigation into the influence of the degree of fluorination on the static and dynamic wetting behavior of TiO₂‐based nanobelt (TNB) particles with various liquids is described. The effect of the degree of fluorination and the surface tension of the liquid on the occurrence and stability of liquid marbles, foams or dispersions are studied and the wetting behavior and arrangement of particles at the air–liquid surface are observed. Using contact angle (θ) measurements, the relation between the type of particle‐stabilized material and θ is established. For liquids of relatively high tension like water or formamide which do not wet the fluorinated particles, a powder‐like material (marble) is formed. For polar oils of intermediate tension (35–50 mN m^?1), which partially wet the fluorinated particles, stable air‐in‐oil foams can be prepared in which particles form a close‐packed layer enveloping air bubbles. Liquids of relatively low tension, e.g., ethanol or polydimethylsiloxane, wet the particles forming a uniform dispersion and partial sedimentation. By contrast, the as‐prepared hydrophilic TNB particles are rapidly wetted by all the liquids as expected due to their high surface energy. The stable cross‐stacked TNB particles with fluoroalkylsilane (FAS) modification could be a versatile platform in a wide range of applications, especially for fluidic devices (e.g., biofluids, gas sensing, and lab‐on‐a‐chip devices). In a proof‐of‐concept study, the oil–water separation performance of fabrics with chemically stable TNB/FAS coating and the liquid isolation by a TNB/FAS shell for highly sensitive gas sensing or reagent assays are investigated.     

湿颗粒堆力学特性的离散元法模拟研究

利用基于线性黏聚接触模型的离散元法对不同颗粒系统的堆积过程进行了数值模拟研究,分析了颗粒形状和湿颗粒间液桥力对颗粒堆积形态的影响机理,获得了球形和块状湿颗粒堆基底表面所受的法向力以及堆中颗粒间的法向力和切向力"中心凹陷"式的分布规律,讨论了颗粒形状和黏聚能量密度对基底表面作用力和颗粒间作用力的影响.研究结果表明,颗粒形状和液桥力对颗粒堆的堆积形态具有显著的影响.堆积角随着黏聚能量密度的增加而增大,并且相同条件下的块状颗粒堆积角大于球形颗粒.颗粒形状和黏聚能量密度对基底表面所受作用力和堆中颗粒间的作用力变化及最大幅值均有影响作用.当黏聚能量密度值逐渐增大时,颗粒堆的作用力最大幅值均逐渐增大,并且块状颗粒堆的作用力最大幅值大于球形颗粒堆.当黏聚能量密度值过大时,颗粒堆力学特性更加复杂,液桥力对颗粒堆积特性的影响作用大于颗粒形状的影响.     

表面张力对固壁旁空泡运动特性影响的理论和实验研究

表面张力是影响空泡脉动及空蚀的一个重要因素.对五种不同表面张力液体中空泡脉动（膨胀和收缩）过程进行了研究,并将实验结果与基于空泡生长和溃灭理论的计算结果进行了对比.实验中,用激光作为测试光源,采用光偏转测试系统研究了不同表面张力液体中空泡泡壁运动规律及泡壁速度的变化.结果表明：表面张力对空泡膨胀过程起抑制作用,故液体表面张力愈大,空泡能达到的最大直径越小;表面张力对空泡的收缩过程则起加速作用,液体表面张力愈大,收缩越迅速,空泡泡壁运动速度越大,其所产生的瞬时溃灭压强越大,空化效果越好. 关键词： 表面张力 空泡 光偏转     

Hydrocarbons imbibition for geometrical characterization of porous media through the effective radius approach

Surface energetic characterization of porous solids usually requires the determination of the contact angle. This quantity is deduced by imbibition experiments carried out in such media with high surface tension liquids. Now then, this methodology needs the geometrical characterization of the porous medium by means of the deduction of its effective radius. Normally, this is made by imbibition experiments with n-alkanes, liquids whose surface tension is low enough as to suppose their contact angles with the solid surface are null. However, this last procedure is not free from some criticisms. Among them, the possible influence of the imbibition velocity on the contact angle, the effect of the precursor liquid film ahead the advancing liquid front on the driving force that gives rise to the movement, or the dependence of the effective radius on the length of the hydrocarbon chain of the n-alkanes. In an attempt of going deeply in these questions, imbibition experiments with n-alkanes have been carried out in porous columns of powdered calcium fluoride. These experiments have consisted of the measurement of the increase in the weight of the columns caused by the migration of the liquids through their interstices. The analysis of their results has been carried out by means of a new procedure based on the study of the velocity profile associated to the weight increase. This analysis has permitted us to conclude that, at least in the calcium fluoride columns, the contact angle of the n-alkane is not influenced by the capillary rise velocity, it taking in fact a null value during the process. On the other hand, it has been also proved that the driving force of the movement is caused by the replacement of the solid-vapour interface by the solid-liquid interface that happens during the imbibition, which means that only the Laplace's pressure, and not the precursor liquid film, contributes to the development of the phenomenon. Finally, it has been compared the values of the effective radius associated to each n-alkane, similar values being found independently from the particular liquid employed in the experiments, fact that indicates that the porous solid can be considered as a bunch of cylindrical and parallel capillaries of the same radius.     

Dipole-dependent slip of Newtonian liquids at smooth solid hydrophobic surfaces

We present atomic force microscopy observations of the "effective" slippage of various nonpolar and polar liquids on alkylsilane coated glass surfaces. For small contact angle nonpolar liquids, the slip length decreases as one approaches a wetting transition. However, for large contact angle polar liquids it is found that the slip length is primarily influenced by the dipole moment, rather than the wettability of the liquid for the surface, where the slip length decreases with increasing dipole moment.     

微重力条件下与容器连通的毛细管中的毛细流动研究

利用落塔的短时微重力条件, 实验研究了与容器连通的毛细管中的流体在微重力条件下的毛细流动过程, 并通过理论分析建立了相应的毛细管中弯月液面高度随时间变化的微分方程. 结果表明, 对于不同的接触角和不同的容器/毛细管参数, 由建立的理论公式得到的数值解结果都与实验结果在定量上较为一致. 此外, 实验中发现, 改变乙醇和去离子水混合液的比例可以明显地改变接触角参数, 但对毛细流动的影响很小, 建立的理论公式也对这一现象给出了合理的解释. 该研究对于预测和分析微流道及空间微重力条件下的毛细流动行为具有明显的应用价值.     

Predicting the Performance of Granulation Binders Through Micro‐Mechanistic Observations

The selection of an appropriate polymeric binder to be used to agglomerate drug with excipients is a critical issue for the development of high shear wet granulation processes for pharmaceutical tablet systems. The aim of the study reported here is to determine the potential for successful granulation through measurement of the interactions of two polymer binder solutions, hydroxypropyl methylcellulose (HPMC) and polyvinyl‐pyrrolidone (PVP), with individual paracetamol drug crystals. A novel micro‐force balance (MFB) has been used to measure different parameters of the crystal‐to‐binder interaction, including the forces exerted by axially strained liquid bridges formed between either two paracetamol crystals or between a reservoir of binder solution and a single paracetamol crystal, the paracetamol‐to‐binder wettability, the post rupture volume distribution and the residue deposited on each crystal. Video images of the separation sequences were obtained simultaneously for analysis of bridge geometry, contact angles, volume distribution and binder residues. It was found that the formation of liquid bridges and their ability to bond particles together depends on the amount of binder left on a crystal after contacting a reservoir of binder (for example, large binder drop). Crystals in contact with HPMC were able to retain more liquid from a binder reservoir than those contacted with PVP solutions. This behaviour is seen to be more important to the final granule strength than the liquid binding force holding particles together in the wet agglomerate.     

Relationship between Dispersive Surface Tension and Density and Molecular Weight of Solvents and Polymers

The surface and interfacial properties of polymers are important for their applications. Generally, the surface property is quantitatively characterized by the surface tension or surface tension component parameters, which are obtained with the contact angle technique. However, the contact angle technique has an inherent problem, contact angle hysteresis phenomenon, which will result in many incredible surface tension data. In order to find a simple and easy method to estimate the rationality of a surface tension result, the relationship between dispersive surface tension component and density and molecular weight is researched in this work. It is found that for 30 organic solvents, there is a good relationship between the dispersive surface tension r^d and the parameter . In addition, for 12 polymers, when the molecular weight is replaced with the molecular weight of the repeat unit, there is still the same relationship as for small liquids. However, because it is difficult to judge the accuracy of the published dispersive data of the polymers, the found experiential relationship needs further confirmation, requiring more reliable published data.     

Phase field modeling of liquid jets in low gravity

An axisymmetric phase field model is developed and used to model surface tension forces on liquid jets in microgravity. The previous work in this area is reviewed and a baseline drop tower experiment selected for model comparison. The model is solved numerically with a compact fourth order stencil on an equally spaced axisymmetric grid. After grid convergence studies, a grid is selected and all drop tower tests modeled. Agreement was assessed by comparing predicted and measured free surface rise. Trend wise agreement is good but agreement in magnitude is only fair. Suspected sources of disagreement are the simple turbulence model and the existence of slosh baffles in the experiment that were not included in the model. Parametric investigation was conducted to study the influence of key parameters on the geysers formed by jets in microgravity. Investigation of the contact angle showed the expected trend of increasing contact angle increasing geyser height. Investigation of the tank radius showed some interesting effects and demonstrated the zone of free surface deformation is quite large. Variation of the surface tension with a laminar jet showed clearly the evolution of free surface shape with Weber number. A breakthrough Weber number of 1 was predicted by the variation of the surface tension model which is close to the experimentally measured Weber number of 1.5 found in prior experimental work.     

Dynamic forces on agglomerated particles caused by high-intensity ultrasound

In this paper the acoustic forces on particles and agglomerates caused by high-intensity ultrasound in gaseous atmosphere are derived by means of computational fluid dynamics (CFD). Sound induced forces cause an oscillating stress scenario where the primary particles of an agglomerate are alternatingly pressed together and torn apart with the frequency of the applied wave. A comparison of the calculated acoustic forces with respect to the inter particle adhesion forces from Van-der-Waals and liquid bridge interactions reveals that the separation forces may reach the same order of magnitude for 80 μm sized SiO₂-particles. Hence, with finite probability acoustically agitated gases may de-agglomerate/disperse solid agglomerate structures. This effect is confirmed by dispersion experiments in an acoustic particle levitation setup.     

纳观接触角的确定方法

对纳观接触角的确定曾有过许多研究工作, 本文对各种理论进行分析评论, 指出其各自的优缺点甚至错误, 认为最为简单实用的理论是朱如曾于1995年在《大学物理》((Vol. 14(2))) 的文章中对前人的宏观接触角的错误理论采用澄清接触角概念的方法所得到的纳观接触角的近似理论及近似公式α = (1-2E_PS/E_PL)π (其中E_PL和E_PS分别表示液体内部一个液体分子的势能和固体表面一个液态分子与固体的相互作用势能, 并可用分子动力学(MD) 模拟得到), 此理论属于纳观接触角的分子动力学理论的近似简化形式, 值得进一步发展. 为此, 本文根据物理分析假设Gibbs张力表面上位于非三相接触区的一个液体分子的势能为E_PL/2x, 三相接触线上一个液体分子与其余液体的相互作用势能为(1+kE_PS/E_PL)α E_PL/2xπ, 其中x和k 为优化参数. 根据Gibbs分界面上处处势能相等条件, 得到改进的纳观接触角的近似公式α = π({1-2xE_PS/E_PL)/(1+kE_PS/E_PL)．对固体表面的氩纳米液柱, 在温度90K下对液体分子之间采用林纳德－琼斯(L-J) 势, 液体分子与固体原子间采用带有可变强度参数a的 L-J 势, 对0.650< a <0.825 范围内的8种a值进行了MD模拟．得到了相应的Gibbs 张力面．将其纳观底角视为近似纳观接触角, 结合物理条件(当E_PS/E_PL=0时, α = π)用最小二乘法得到优化参数值x=0.7141, k=1.6051和相关系数0.9997. 这一充分接近于1的相关系数表明, 对于不同相互作用强度的纳米液固接触系统, 优化参数x和k确实可近似视为常数, 由此确认我们提出的利用MD模拟来确定纳观接触角近似公式中优化参数的可行性和该近似公式的一般适用性.     

Production of Fine Particles from Melts of Metals or Highly Viscous Fluids by ultrasonic standing wave atomization

Ultrasonic standing wave atomization (USWA) is a new process capable of atomizing both high surface energy liquids and highly viscous liquids. Atomization is achieved through acoustic forces acting upon a liquid jet which is guided into the central pressure node of a standing wave field. Spherical metal powders with minimum mass median diameters of less than 15 μm have been produced from metal melts with surface tensions of about 0.5 N/m. Organic liquids with viscosities between 1 and 10 Pas have been atomized, yielding mass median diameters from 20 to 330 μm. The influence of different operating parameters on the mass median diameter of metal melts and highly viscous liquids was evaluated. Parameters which were varied were ambient gas pressure, vibration amplitude of the transducers, mass flow rate, density of liquid, viscosity of the liquid, surface tension and the outlet diameter. The powders and sprays were analyzed with laser diffraction particle sizers. The physical background of the atomization process is discussed and an equation for the prediction of the mass median diameter is derived.     

Solvent‐Assisted Interfacial Tension Deformation of Spherical Particles for the Fabrication of Non‐Spherical Particle Arrays

A facile and efficient approach is developed for the fabrication of asymmetric non‐spherical polymer particle arrays. A specific amount of solvent is provided to interact with the spherical polymer particles to intensify the segmental mobility, thus suppressing the viscosity and the glass transition temperature of the polymer particles. The spherical polymer particles in the rubbery state are deformed into non‐spherical particle arrays at the gas/liquid interface. The upper parts of the polymer particles that protrude out of the liquid phase undergo deformation by interfacial tensions at the three‐phase contact line, allowing the formation of a ridge of polymer with a protrusion on the top surface. Simultaneously, the lower parts of the polymer particles submerged under the liquid phase are subjected to enormous surface tension at the contact points, leading to a non‐linear coalescence behavior of the neighboring polymer particles.     

Measures of wettability of solid surfaces

The surface tension of a solid surface is not amenable to direct experimental measurement. The most common method for assessing this surface tension is by contact angle measurements. The currently optimal way to measure and interpret contact angles is discussed, emphasizing the yet unresolved issues. It is argued that the most meaningful measurements to be done are of the most stable apparent contact angle (from which the surface tension of the solid is eventually assessed) and the contact angle hysteresis range (which indicates the existence and degree of chemical heterogeneities and roughness).     

Contact electrification phenomena on phosphor particle surfaces

Contact electrification phenomena are reviewed to understand phosphor powder behavior. The origin of contact electrification is described in terms of the acid–base concept for solid surfaces. Electric charges produced by contact electrification affect phosphor properties. Various examples related to contact electrification are shown: particle surface modification, dispersion in liquids, adhesion strength to a substrate, electrostatic coating, interaction between a phosphor and mercury in a fluorescent lamp, and electron emission ability in a fluorescent lamp.     

The field of radiative forces and the acoustic streaming in a liquid layer on a solid half-space

The acoustic field and the field of radiative forces that are formed in a liquid layer on a solid substrate are calculated for the case of wave propagation along the interface. The calculations take into account the effects produced by surface tension, viscous stresses at the boundary, and attenuation in the liquid volume on the field characteristics. The dispersion equations and the velocities of wave propagation are determined. The radiative forces acting on a liquid volume element in a standing wave are calculated. The structure of streaming is studied. The effect of streaming on small-size particles is considered, and the possibilities of ordered structure formation from them are discussed.     

Preparation of hybrid film with superhydrophobic surfaces based on irregularly structure by emulsion polymerization

A superhydrophobic surface originated from quincunx-shape composite particles was obtained by utilizing the encapsulation and graft of silica particles to control the surface chemistry and morphology of the hybrid film. The composite particles make the surface of film form a composite interface with irregular binary structure to trap air between the substrate surface and the liquid droplets which plays an essential role in obtaining high water contact angle and low water contact angle hysteresis. The water contact angle on the hybrid film is determined to be 154 ± 2° and the contact angle hysteresis is less than 5°. This is expected to be a simple and practical method for preparing self-cleaning hydrophobic surfaces on large area.     

Towards a Fundamental Understanding of the Mechanics of Crystal Agglomeration: A Microscopic and Molecular Approach

A novel experimental apparatus has been developed which enables the measurement of adhesion forces between two crystals suspended in a supersaturated solution and allowed to agglomerate over a fixed time period. The geometry of the crystal surfaces at the contact points and the dynamic development of the bond are captured on video and characterised using an image analysis technique. The experimental apparatus has been designed to allow control of supersaturation, orientation of crystal faces, distance between crystals, relative movement of crystals and contact time. The experimental results show that the agglomerate bond strength, expressed as the agglomerate adhesion force per unit contact area, increases with increasing supersaturation and is higher for faster growing faces than for slower growing faces. In addition, a qualitative comparison has been made between the measured force and a theoretical estimation of the interaction force between crystal faces, determined through molecular modelling. It is shown that the speed of approach of two opposing crystal faces is a key parameter in the nature of the subsequent bond, as is their atomic structure.     

Determination of surface tension coefficient and contact angle using numerical calculations of equilibrium drop shapes

The technique of determination of the contact angle and surface tension coefficient based on numerical calculation of the geometric characteristics of equilibrium shape of a liquid drop on the horizontal surface is stated. As initial data, the technique suggests using the dimensionless values of drop height and contact-spot radius that are obtained from drop images.