Advanced Ultrasonic Monitoring for Concentrated Dispersions and Nanoparticle Materials

当平面超声波(频率高于人类所能听到的声波)通过悬浮在其中的颗粒或液滴的流体时，颗粒/液滴通过向其他方向发送部分波来散射波。当阳光被空气中的水滴散射时，一种非常类似的效应会产生彩虹。对于超声波，这是一种压缩波，颗粒的散射也可以将一些波转换为其他类型的波，即热波和剪切波。这些过程带走了超声波的能量，从而导致其振幅降低。通过测量超声波通过悬浮液的衰减(幅度损失)和波速，我们可以找出颗粒的浓度，它们有多大，或者关于它们的性质的东西，如它们的密度。由于这些类型的材料(颗粒悬浮液)具有多种用途，例如食品、保健品、农用化学品、药物输送系统，因此测量其性能的方法是生产过程中的关键要素，在许多行业中具有重要意义。为了理解我们所做的测量，我们需要使用一个模型、一组方程和计算，告诉我们粒子和流体的性质如何影响波的幅度和速度的损失。我们使用的模型有两个部分：多重散射理论和单个粒子的散射模型。一段时间以来，我们使用的模型一直是有限的，因为它对粒子产生的另外两种波(热波和横波)进行了一些近似；它假设这些波在非常短的距离内消失，对附近的粒子没有任何影响。虽然它们确实在很短的距离内消失，但当悬浮液非常集中(即在一个很小的空间中有许多粒子)时，它们会影响邻近的粒子。热波和横波本身可以被附近的粒子散射，并可能部分转换回纵波(超声波)。这意味着我们从纵波中损失的能量并不像我们想象的那样多。波的转换和再转换的过程被称为多模散射，多年来，由于我们没有合适的模型来计算它，它的影响被忽略了。去年，法国勒阿弗尔的一组研究人员在最初的多重散射模型发表40多年后，发表了新版本的多重散射理论，其中确实包括了这种多模散射。这是一个有用的发展，但目前该模型是一组相当抽象的数学方程，其中包括许多我们还不知道如何计算的项。我们打算做的是将这种模型转换成一种能够在线监测管道中的超声波的形式。我们将计算出这些方程中的哪些部分对典型悬浮液中测量的超声波速度和衰减(能量损失)影响最大。我们将开发一些新的单个粒子散射模型，以便我们可以计算出波模之间转换了多少能量。这些模型将采取将由计算机求解的方程组的形式(数值模型)，以及一些可以直接写成数学符号的形式(分析模型)。为了证明所建立的模型是有效的，将对相对高浓度(体积分数为10-30%)的悬浮液中的衰减和波速进行实验测量，并在一定的颗粒尺寸范围内进行测量。该项目的成果将是一种可用于在线超声仪器的模型。这将使超声波能够在广泛的工业环境中作为一种过程监控技术被自信地使用。

When a plane ultrasonic wave (a sound wave at higher frequency than humans can hear) travels through a fluid which has particles or droplets suspended in it, the particles/droplets scatter the wave by sending some of it in other directions. A very similar effect produces a rainbow when sunlight is scattered by water droplets in the air. With ultrasonic waves, which are compressional waves, scattering by the particles can also convert some of the wave into other wave types, namely thermal and shear waves. These processes take energy away from the ultrasonic wave which causes a reduction in its amplitude. By measuring the attenuation (loss in amplitude) and the wave speed for an ultrasonic wave travelling through the suspensions, we can find out the concentration of particles, how big they are, or something about their properties e.g. their density. Since these sorts of materials (suspensions of particles) have many uses e.g. foods, healthcare products, agrochemicals, drug delivery systems, a way of measuring their properties is a crucial element of a production process and of great importance in a number of industries. In order to understand the measurements we make, we need to use a model, a set of equations and calculations which tell us how the properties of the particles and fluid affect the loss of amplitude and speed of the wave. The model we use has two parts: a multiple scattering theory, and a model for the scattering from a single particle. For some time, the model we used has been limited because it made some approximations about the two other wave types (the thermal and shear waves) which are produced at the particles; it assumed that those waves die away in a very short distance, and do not have any effect on the particles nearby. Although they do die away in a very short distance, they can affect the neighbouring particles when the suspension is very concentrated (i.e. there are a lot of particles in a small space). The thermal and shear waves themselves can be scattered by particles nearby and may be partly converted back into a compressional wave (an ultrasonic wave). This means we did not lose as much of the energy from the compressional wave as we thought. The process of wave conversion and re-conversion is referred to as multi-mode scattering and for many years, its effect has been ignored because we did not have a suitable model to calculate it. Last year, a group of researchers at Le Havre (France), published a new version of the multiple scattering theory, which does include this multi-mode scattering, over 40 years after the original multiple scattering model was published. This is a useful development, but at the moment the model exists as a set of rather abstract mathematical equations which include many terms which we do not yet know how to calculate. What we propose to do is to transform this model into a form which enables online ultrasonic monitoring in a pipe. We will work out which parts of those equations make the most difference to the measured ultrasonic speed and attenuation (energy loss) in typical suspensions. We will develop some new models for scattering by a single particle so that we can work out how much energy is converted between wave modes. These models will take the form of sets of equations which will be solved by computer (numerical models), and also some forms which can be written directly in mathematical notation (analytical models). To demonstrate that the models developed in the project are valid, experimental measurements will be made of the attenuation and wave speed in suspensions at relatively high concentrations 10-30% by volume, and for a range of particle sizes. The outcomes of the project will be a model in a form which can be used in online ultrasonic instrumentation. This will enable ultrasonics to be used with confidence as a process monitoring technique in a wide range of industrial contexts.