Acoustic Wave On Soft Matter

Introduction

This chapter critically reviews existing literature on the soft matter, properties and behaviour as well as applicability in different fields as discussed and hypothesised by previous researchers. Furthermore, the chapter covers the effect of acoustic wave on soft matter like liquid crystals, polymers, and colloid as well as types of liquid crystal and nemetic and smectic phase. Lastly, the literature review into acoustic wave on soft matter explores the missing gaps in the properties of LC and colloid.

2.1 Introduction of Soft matters

Kleman and Laverntovich described soft matter as generalised termed used for materials that “easily deformed by thermal fluctuations and external forces” [1]. Also widely referred to as soft condensed matter, it comprises of materials that can be easily deformed or structurally altered with exposure to mechanical or thermal stress. As pointed by Sagis, the deformation of the soft caused by external forces or thermal fluctuations occurs at a low temperature, an energy scale comparable with a room temperature thermal energy. As such, as pointed by Pierre-Gilles de Gennes, who pioneered research on soft matter and application of methods from condensed-matter physics to liquid crystals and polymers, properties such as magnetic dipoles or molecule chains of a material can transitions from order to disorder especially for crystals and polymers in solution. Currently, theoretical descriptions of soft matter is derived through classical physics and defined along symmetry breaking and the statistical mechanics, equilibrium and non-equilibrium tools. As pointed by Nagel, the softness characteristics of these materials are derived from the large structural units and relatively weak interactions holding the building blocks as well as responsible of distinct features of materials such as sensitivity to external stimuli (strain and thermal fluctuations), non-equilibrium states, and non-zero flow behaviour [3]. Unlike hard-condensed matter characterised by regular crystalline lattice resulting in predictability of it properties, soft matters have intrinsically heterogeneous structure resulting to complex interactions and slow dynamics [13]. According to Kitto, the interrelation between thermal fluctuations and interactions can result with complex emergent behaviour like spontaneous pattern formation, large response to small external stimuli, and self-assembly [14].

2.2 Properties of Liquid crystal (LC) colloids and emulsions

Liquid crystals are material with properties that lie between liquids and solids. The LC materials exhibit the characteristics behaviour to flow but its molecules retains the crystalline properties lacking in liquids [4]. In some materials, LC properties occur at intermediate state between liquid state at high temperature and low temperature of crystalline solid when melting. On the other hand, colloid and emulsion are mostly used interchangeably but the former is a homogenous non-crystalline substance formed by dispersion of ultramicroscopic particles or large molecules through second substance. On the other hand, emulsions are mixture of two or more liquids that are not soluble or miscible resulting in one present as droplets of either ultramicroscopic or microscopic size [5, 6]. Emulsion can either be stable or unstable where the former separating eventually into separate layers whereas stable emulsions can be separated or destroyed by inactivating emulsifying agents, for instance heating, freezing, or adding third substance. Unlike liquid with positional order, the molecular orientational order (director) of a liquid crystal are elongated and points to the average direction of the molecules, as shown in figure 1. The liquid crystal materials show several phases that include nematic, smectic, and columnar depending on the molecular structure, thermodynamic parameters variations (concentration, temperature), and intermolecular interactions. In any system, the amount of order present is described by order parameter (S). In a nematic liquid crystal, the order parameter is expressed as

Equation 1

Binks and Horozov highlighted that colloidal particles can be combined synthetically in various shapes and sizes while being capable of tuning the interactions between them [7]. In colloidal phase behaviour, the material formations take a bottom-up approach starting with nanometre-sized colloidal building blocks. For example, changing such characteristics as salt concentration, decorating the particles using polymers, varying the surface charge, or adding polymeric additives. As such, the resultant can be of a larger structure and phases based on self-organization taking the shape and characteristics of the buildings blocks. For example, glasses, liquid crystals, colloidal liquids, and crystals take behaviour of its fundamental blocks, atoms and molecules. Poon illustrated that the big and slow features of colloids where one can observe individual component using optical microscope can be useful in studying fundamental physics problems [8]. Such characteristics and properties as crystal defects, melting and nucleation can be studied at individual particle level.

2.4 Properties and Application of soft matter

Application of soft matter

Colloids can be designed by modelling its building blocks to become fundamental elements ‘molecules’ of the future materials with potentially distinct and varying catalytic, mechanical, and optical properties. However, according to van der Gucht, challenges related to self-assembling such as ways of pre-programming desired structure into individual molecule [13]. Secondly, most colloids interactions are isotropic limiting the ranging of structure lattice that can be formed with core building blocks like body- and face- centred cubic crystals. Obtaining other symmetries, it demands particles with anisotropic shape and interactions potential. Although, currently, several approaches have been forwarded on preparing such particles, there has been several limitations in successful implementations including assembling the crystal structures. Moreover, it is possible to simulate virtually implementation of potential interactions, designing the building blocks informing the colloids imposed a huge difficulty particularly with precisely specified interaction [15]. Lastly, compared with molecular self-assembly, colloids have more non-equilibrium states. However, by using gelation process, the particles can be restricted in a non-equilibrium state aggregated state resulting in completed suppressed crystallization.

In study on the gelation of particles with short-rage attraction, Lu et al. pointed that the potential of colloids to change the properties of materials taking a various behaviours ranging from complex liquids to solid-like characteristics have significant applications in technology and science [17]. Particles aggregation based on antiparticle attraction and leading to formation of mesoscopic networks and clusters can result in diffusion-limited cluster aggregation (DLCA) in the event of limited irreversible aggregation. The formation is taken as purely kinetic phenomenon and, at arbitrarily low particle volume fractions; it can result in formation of solid-like gels.

In mixed systems, emulsion is widely applicable to liquids better characterised as gels, suspensions, or solutions. Emulsions take liquid state and do not have internal structure. Moreover, most emulsions are unstable with microscopic structures that are not soluble remaining suspended indefinitely. Study conducted by McClements on formation and properties of liquid crystal emulsion in oil-in-water, it demonstrated that liquid crystal structure in oil-water forms gradually with cooling process [19]. Investigating crystallisation in emulsions, Coupland stated that successful development emulsion consisting of small oil droplets dispersed in water depends on a good comprehension of the influence of crystals on properties, behaviour, stability, and formation of emulsion [20].

2.5 Acoustic waves –types and applications

The acoustic waves are type of energy propagation considered mechanical and longitudinal compression and decompression resulting from oscillation of pressure travelling through medium (gas, liquid, or solids). The waves exhibit several properties that include frequency, wavelength, amplitude, and period. According to Friend and Yeo, acoustic wave properties are described by the quantities such as particle velocity, particle displacement, acoustic pressure, and acoustic intensity [21]. However, as pointed by De Ryck et al. these quantities and properties fundamental are subject of vibration of building blocks of a medium [22].

Notably, studies have demonstrated recognizing sound and wave interaction with structural order of liquids resulting in repositioning of liquid-crystal molecules. The resultant repositioning of the molecule is transmission of optical intensity through the crystal structure. Building from Collings and Goodby assertion, description of the liquid-crystal where the properties of solids are combined with those liquids, flow properties, taking advantage of useful properties and behaviour of both the solids and liquid has provided foundation of low-power revolution in display technology (flat-panel liquid-crystal display (LCD)) [24]. Apparently, the characteristic velocity of the wave largely relies on medium in which it propagates through such as earthquake wave travelling through earth crust and resulting in ground movement. In liquid, acoustic wave propagate through vibrations of particles through either molecules moving forth and back ‘longitudinal waves’ or acting perpendicular to the direction of propagation ‘transverse waves’ [24]. The acousto-optic effect caused by coupling between acoustic intensity and variations in the optical transmission has been found to be useful in technological applications because of its capability of visualization through direct liquid-crystal. As pointed by Selinger et al. interactions liquid crystals and sound wave tend to be complex in high-intensity wave that can result in shear flow in the crystal cell while taking a simpler format in low intensity [25].

Transverse Waves

Transverse waves are regarded as acoustic waves that have their particles vibrate perpendicular to the significant direction of the wave. In the process, the material is said to undergo the shear deformation. Transverse acoustic waves can only propagate in solids alone.

On ultrasonic interaction with nematic liquid crystal, Dion and Jacob hypothesised that torque tend to align liquid-crystal molecules perpendicular to the vibrations caused by acoustic waves. Therefore, the torque is relatable to the anisotropic attenuation of ultrasound and minimizing propagation losses. As highlighted by Penciu et al. brilloum light scattering can be used in detection of vibrational modes in suspensions of soft colloids showing a ‘opticlike’ mode at a low volume fractions in additional to usual longitudinal phonon of the liquid matrix [26]. Apart from taking note of the types of acoustic waves, it is also significant to take note of the speed of sound, which is an essential property. It is worth noting that there is a strong relationship across the speed of sound, the wavelength and frequency as show below.

However, it should be noted that the speed of the sound would vary based on the media. This means that speed of sound in any medium can be determined with help of density and medium rigidity. If the media is more rigid, then this means that the speed of sound would be faster. However, density of the media would still slower the same speed. Based on this property, the speed of sound can be extracted from the acoustic wave equation, which presents the mathematical description of the acoustic pressure. If p represents the acoustic pressure, t is time and x is the distance, then it follows that

Studies around the speed of found have further touched on material characterization. The mode of propagation would yield four types of the ultrasonic velocities, namely lamb, shear, surface and longitudinal wave velocity as indicated by Pandey and Pandey [30]. This means that profound mechanical properties associated to solids would largely differ from the ones in solids. The first difference appears in terms of the binding forces across the constituent atoms for the purposes of supporting shear stress. The second difference is determined by anisotropy, which is evident in single crystals where the atoms would form the regular lattice. Based on these two properties, it is evident that the velocity of the ultrasonic wave can be computed from the Poisson’s ration (σ), the modulus of rigidity (G) and the Young’s modulus (Y) and density d as well [31]. Therefore the shear velocities Vs and the longitudinal velocity VL can be expressed as

Apart from the equations determining the speed of sound, acoustic waves have a wide range of applications as indicated by Huang et al [33]. The acoustic waves are evident in most of the musical instruments. In this case, sound waves are regarded as vibrations that would produce sound. Stringed instruments like harp, guitar or even the piano includes setting the strings into vibration. Horns and woodwind instruments would produce sound through the mouthpiece. In this case, vibrations emanate from the sound effects. Kustov et al [32] believes that acoustic waves are taking a significant position in electronic amplification. Most of the machines or automobiles would produce sound during the operation. Sound is also a significant form of energy and can still be converted into other significant forms of energy and this is what normally happens in the functionality of microphones. The microphone would receive the sound waves before converting them into electrical energy.

The signals would then be transmitted to the amplifier and subsequently to the loudspeaker, which further converts the electric signals into sound again. The loudspeaker has a thin flexible disk and a diaphragm, which would vibrate based on the intensity of the produced sound. Human voice has equally been tagged into the applications of acoustic waves as far as speeches are put into consideration. Acoustic waves are also widely used in the acoustic imaging system. Chu et al [34] asserted that acoustic systems such as the echo-sounders have been utilized for around 80 years in finding fish. The echo-sounders are believed to transmit a given pulse of the sound and further receive a backscattered acoustic energy from the targets. When the speed of sound is high, while the density contrasts the object, there will a louder echo. Notably, understanding how the backscattered energy relies on the morphometry, anatomy as well as material properties of fish can be relevant in the study of acoustic technology [38]. Besides, the use of high frequency acoustic imaging model can enhance the capacity of facilitating more details regarding visualization of the scattering structure attached to the morphology of the fish.

Again, the virtual acoustic imaging models or systems are also gaining space when it comes to application of the acoustic waves. The system depends on the design of the significant matrix associated to the inverse filters in which the transfer functions are linked to the loudspeaker input signals [47]. Notably, inverse filter matrix is essentially used in operating on signals which are desired by ears of the listener for the purposes of producing appropriate input signals. Essentially, the design of the filters is regarded as the key factor while ensuring that the systems take into consideration the most basic geometry. The movements of the system need to be robust due to the critical role played by the geometry of the loudspeaker or the listener [39]. Optimization of the geometry is therefore regarded as a significant process in the course of reproducing the desirable signals [36]. The inverse matrix needs to compensate to all the movements made by the listener’s head. Another application is seen in the acoustic borehole imaging. Micro-sensitive scanners are commonly used in this case.

2.6 The effect of acoustic wave on LC, polymer, colloid

The study of acoustic waves has been linked to its effect on first, the liquids and solids. El Boudouti et al [35] asserted that acoustic waves or sound waves liquids would travel faster compared to gasses. In fresh water, the waves would travel at 1482 m/s. Most of the ocean dwelling animals depend on the sound waves in communicating with others as regards location of food as well as obstacles. The speed of the acoustic waves in liquids is expressed as

Boudouti et al. took a deeper observation of the acoustic waves in the fluid and solid layered materials. The acoustic wave propagation across the layered materials includes the elastic, piezoelectric as well as viscoelastic layers. Notably, the phonon modes are largely emphasized as far as the periodic multilayered structures are put into consideration. In the course of propagating bulk waves, attention is paid towards the impact of inhomogeneities across the perfect superlattice like the defect layer, the substrate interface and the free surface. The defect layers, according to Shen and Zhang, give room for wave filtering and these would introduce the media to imperfection especially when there is an epitaxial growth process [36]. The substrate interface is most likely to exhibit the interface modes. The resonant modes would easily be realized when interacting with bulk waves, surface and interface waves. Further integration of the dynamic photo-elastic method and Schlieren method led to the realization that both solids and liquids can experience the surface wave, shear wave and even the compression wave [48]. While using the liquid and aluminium, Shen and Zhang found out that one can easily establish a coupling relationship linked to the sound wave via the interface.

3.2 The effect of acoustic wave on liquid crystals

More studies have also been aligned to the effects of the acoustic waves in the liquid crystals. According to Bury et al., liquid crystals are believed to have attracted attention of the researchers due to the exceptional electro-optical as well as thermo-optical properties [37]. Acoustic or ultrasonic methods are largely applied for the viscous parameters and characterization of the LC elasticity. Notably, surface acoustic wave would essentially determine the significant viscosity distribution across the LC layer based on the electric field. Due to the acoustic waves, there would always be the impact of structural transformation as a result of acoustic oscillations [40].

Destgeer et al. provided insights into the propagation of the acoustic waves in the complex colloidal systems [43]. This nature of propagation is likely to attract new properties, which would characterize both the dynamics as well as the structure of the suspension. Notably, the presence of colloidal particles across the suspension would probably lead to structures that exhibit length scales which go beyond the molecular size [41]. Studies have been directed towards the behaviour of microemulsions and micelles. Dispersion would be measured as part of the volume fraction of the microemulsions and micelle, which can still be varied [44].

Upon introduction of the acoustic waves, it is evident that the sound velocity within the system would increase as volume fraction of the droplets increase. This is an obvious pointer to the fact that surfactant shells linked to the droplets would be more rigid compared to the continuous oil medium. However, the variability in terms of sound velocities at both high and low frequencies would still facilitate the measure of the augmented shear modulus as a result of the networks [42]. Subsequently, the additional rigidity would exhibit the most power law scaling while the volume fraction would be approaching the critical value as part of the characteristic of the percolation behaviour.

3.4 The effect of acoustic wave on polymers

Further attention is given to the relationship between acoustic waves and polymers. Lionetto and Maffezzoli asserted that ultrasonic wave propagation would attract transitions both in linear as well as cross-linked polymers [45]. There is significantly high sensitivity of the ultrasonic waves to the probable changes in terms of the viscoelastic properties associated to the polymers. Powerful approaches have been introduced in studying the necessary polymer transitions. Some of these methods include crystallization, glass transition as well as melting. Based on a range of the studies relating polymers to acoustic waves, it can be established within the glassy state identified in the spectrum region, chances are high are the ultrasonic velocity would linearly decrease if the temperatures go up [49].

3.5 Acoustic Sensor Devices

In the world of sensor applications, Delory et al. [50] revisited some of the acoustic sensor devices. The authors brought in the A1 and A2 sensors. The A1 sensors are the low power and compact digital hydrophone which are known for having the pre-processed acoustic data embedded in it. This type of sensors is commonly known for enabling the acoustic measurements, as well as characterization of the acoustic sources, the underwater noise and the soundscape sources. The A1 sensors comprise of two AD converters and one transducer. The sensor has a transducer signal which can be pre-amplified by around 20 dB. Notably, the first channel constitute a high pass filter, known as an equalizer, connected before the high gain stage for the purposes of optimizing the signal to the sea noise ratio at relatively low frequency.

The equalizer would essentially settle the dynamic range at relatively at high frequency. On the other hand, the A2 sensors are digital passive acoustic transducers in which the output is a pre-processed master unit. The acoustic array constitutes the four units referred to as A2hyd. A2 sensors have a smaller internal memory compared to A2 sensors, and they also have hydrophone JS-B100. Any of the A2hyd devices in the A2 sensors has the capacity of availing the acoustic data with the help of the Ethernet protocol and the serial digital port. Essentially, the Master Unit would manage the synchronization of the A2hyd for the purposes of attaining the simultaneous sampling. Commonly, time synchronization of the slave units and the master unit can be attained with the help of IEEE1588 Precision Time Protocol. The latter is known for defining the network protocol. The most critical role played by the Master Unit includes the post-processing needed for the acoustic data emanating from digital hydrophones A2hyd.

Giangu et al. [51] also highlighted significant acoustic sensing devices, which include Film Bulk Acoustic Wave filters (FBAR) and Surface Acoustic Wave resonators (SAW). The sensors are commonly known for humidity, gas, temperature and pressure sensing. The sensing applications are based on high resonance frequencies due to the fact that sensitivity is essentially proportional to the square of the resonance frequency when it comes to mass, gas and humidity detection. The characteristics of sensors can sometimes be performed on the wafer which is commonly not influenced by the known external circuit elements. This is due to the radio frequency calibration process with the help of contact points associated to the measured device.

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