Cavitation – A novel technique for making stable nano-suspensions

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Abstract

The purpose of the present study was to obtain nano-scale particles of styrene butadiene rubber. As SBR particles are elastic in nature, conventional methods of size reductions such as impacting, grinding are unable to achieve the final size. So, attempts have been made here to make the nano-particles of the SBR using cavitation technique. Both acoustic and hydrodynamic cavitation techniques have been employed and studied. Hydrodynamic cavitation has been proved to be more energy efficient than the acoustic cavitation on the basis of various parameters. The maximum production rate equivalent to 2 kg/h (solid processing) has been achieved in the newly developed hydrodynamic cavitation set-up (made in house). Similar to transient cavitation, stable cavitation has also been shown to contribute for reduction in the size of the material with very low variation in size. This technique has been proved successful for the size-reduction of the elastic material to nano-scale, thus it may also be used for the size-reduction of the other brittle and hard material by adjusting various cavitational parameters.

Introduction

Nano-suspensions have emerged as a promising strategy for an efficient delivery of hydrophobic drugs because of their versatile features such as very small particle size. Techniques such as media milling and high-pressure homogenization have been used commercially for producing nano-suspensions [1]. Recently, the engineering of nano-suspensions employing emulsions and microemulsions as templates has been addressed in the literature [1]. The unique features of nano-suspensions have enabled their use in various dosage forms, including specialized delivery systems such as mucoadhesive hydrogels. Rapid strides have been made in the delivery of nano-suspensions by parenteral, peroral, ocular and pulmonary routes. Currently, efforts are being directed to extending their applications in site-specific drug delivery [1].

In the present study the attempts are made to make a nano-suspension of the styrene butadiene rubber latex in aqueous phase. The major applications for the rubber latex suspension in the nano-scale are

  • Calibration of the instruments.

  • Inhalations studies.

  • Characterization of the particles of the other substance.

  • Density measurement.

The ability to produce the nano-particles of desired size with great precision (narrow size distribution and small variation) is the key factor of producing the nano-suspensions. The process of producing nano-particles can be categorised by two approaches:

  • The top–down approach – where one starts with the bulk material and machines it, way down to the nano-scale.

  • The bottom–up approach, starting at the molecular level and building up the material through the small cluster level to the nano-particle and finally the assembly of nano-particles.

Cavitation is the phenomenon of sequential formation, growth and collapse of millions of microscopic vapour bubbles (voids) in the liquid. The collapse or implosion of these cavities creates high localized temperatures roughly of 14 000 K and a pressure of about 10 000 atm or results into short-lived, localized hot–spot in cold liquid. Thus, cavitation serves as a means of concentrating the diffused fluid energy locally and in very short duration, creating a zone of intense energy dissipation [2].

Cavitation is induced by passing high frequency (16 kHz–100 MHz) sound waves, i.e. ultrasound through liquid media. When ultrasound passed through the liquid media, in the rarefaction region local pressure falls below the threshold pressure for the cavitation (usually the vapour pressure of the medium at the operating temperature), millions of the cavities are generated. In the compression region the pressure in the fluid rises and these cavities are collapsed. The collapse conditions are dependent on the intensity and frequency of the ultrasound as well as liquid physical properties, temperature of the liquid and the dissolves gases [3].

Hydrodynamic cavitation can simply be generated by the passage of the liquid through a specified geometry of constriction such as orifice plates, venturi, etc. When the liquid passes through the constriction, the kinetic energy of the liquid is increasing at an expense of the pressure. If the throttling is sufficient to cause the pressure around the point of vena contracta to fall below the threshold pressure for the cavitation (usually the vapour pressure of the medium at the operating temperature) millions of the cavities are generated. Subsequently, as the liquid jet expands, the pressure recovers and this results in the collapse of the cavities releasing the energy in the form of a high magnitude pressure pulse. During the passage of the liquid through the constriction, the boundary layer separation occurs and substantial amount of the energy is lost in the form of turbulence and permanent pressure drop [4].

Very high intensity of the turbulence, downstream side of the constriction is generated and its intensity depends on the magnitude of the permanent pressure drop, which again depends on the geometry of the constriction and the flow conditions in the liquid. The intensity of the turbulence has a profound effect on the cavitation activity and the intensity as shown by Moholkar and Pandit [5]. A dimensionless number known as cavitation number (Cv) is used to relate the flow conditions with the cavitation intensity as follows:Cv=P2-Pv12ρVo2Here P2 is the recovered downstream pressure; Pv is the vapour pressure of the liquid and Vo is the liquid velocity at the orifice. The cavitation number at which the inception of cavitation occurs is known as the cavitation inception number Cvi. Ideally speaking, the cavitation inception should occur at 1.0. But Harrison and Pandit [6] have reported that, generally the inception of the cavitation occurs from 1.0 to 2.5. This has been attributed to the presence of the dissolved gases in the flowing liquid. Yan and Thorpe [7] have shown that Cv is a function of the flow geometry and usually increases with an increase in the size of the opening in a constriction such as an orifice in a flow.

Advantages of hydrodynamic cavitation over acoustic cavitation have been reported as follows [8]:

  • It is one of the cheapest and energy efficient methods of generating cavitation.

  • The equipment used for generating cavitation is simple.

  • The scale up of the system is relatively easy.

To reduce a material’s particle size, large particles or lumps must be fractured into smaller particles. To initiate fractures, external forces are applied to the particles. Generally, the extent of particle size reduction caused by an external force depends on the amount of energy supplied to the particle, the rate at which it is supplied, and the manner in which it is supplied. The application of size reduction forces can be broken into the following four categories [9].

Impact milling occurs when a hard object that applies a force across a wide area, hits a particle with a certain momentum to fracture it. The least size obtained by an impact mills is of the order of 50 μm for mechanical impact mills and less than 10 μm for fluid jet mills [9].

In attrition milling, non-erodable grinding media continuously contact the material to be ground, systematically grinding its edges down. Attrition mills can reduce 1000 μm (20-mesh) particles of friable materials such as chemicals and minerals down to less than 1 μm. One such type is the media mill (also called a ball mill) [9].

In knife milling, a sharp blade applies high, head-on localized shear force to a large particle, cutting it to a pre-determined size to create smaller particles and minimize fines. Knife mills can reduce 2 in. or larger chunks or slabs of material, including elastic or heat-sensitive materials down to 250–1200 μm [9].

Direct-pressure milling occurs when a particle is crushed or pinched between two hardened surfaces. Direct-pressure mills typically reduce 1-in. or larger chunks of friable materials down to 800 to 1000 μm.

Most mills use a combination of these principles to apply more than one type of force to the particle to be ground. The very important part is to choose the best type of size-reduction mode based on the characteristics of the material to be processed and initial and final size requirements.

The physical properties of the material to be reduced are also important to decide the method and the equipment to be used for reducing it. Non-friable materials such as polymers, resins, waxes, and rubber can’t be shattered or fractured using regular impact or direct-pressure milling. Knife milling often cannot reduce a non-friable material to a very fine particle size range. Typical methods, for reducing non-friable materials require turning the non-friable material into a friable material by freezing it below glass transition temperature. In certain cases, pre-conditioning or exposing the particles to a cryogen may be necessary. For low temperature milling with cryogens, care of the components of the equipment is very important as they also become brittle and certain lubricating greases lose their viscosity and freeze [9].

The extreme transient conditions generated in the vicinity and within the collapsing cavitational bubbles have been used for the size reduction of the material to the nano-scale. Nano-particles synthesis techniques include sonochemical processing, cavitation processing, and high-energy ball milling. In sonochemistry, an acoustic cavitation process can generate a transient localized hot zone with extremely high temperature gradient and pressure [10]. Such sudden changes in temperature and pressure assist the destruction of the sonochemical precursor (e.g., organometallic solution) and the formation of nano-particles. The technique in principle can be used to produce a large volume of material for industrial applications but no reports are available in the open literature.

Use of the cavitation for the formation of the nano-particles has been reported by Suslick [11]. He sonicated Fe(CO)5 either as a neat liquid or in a decalin solution and obtained 10–20 nm size amorphous iron particles. Similar experiments have been reported for the synthesis of the nano-particles of many other inorganic materials using acoustic cavitation [12]. To understand the mechanism of the formation of the nano-particles during the cavitation phenomenon, hot–spot theory has been convincingly used. It explains the adiabatic collapse of a bubble, producing the hot spots. This theory claims that very high temperatures (5000–25 000 K) [12] are obtained upon the collapse of the bubble. Since this collapse occurs in few microseconds (final adiabatic phase), very high cooling rates (in excess of 1011 K/s), have been obtained. These high cooling rates hinder the organization and crystallization of the products. For this reason, in all the cases dealing with volatile precursors, where gas-phase reactions are predominant, amorphous nano-particles have been obtained [12]. While the explanation for the creation of amorphous products is well understood, the reason for the formation of nano-structured products under cavitation is not yet clear. One possible explanation is that the fast kinetics does not permit the growth of the nuclei, and in each collapsing bubble a few nucleation centers are formed whose growth is limited by the short cavity collapse time. If, on the other hand, the precursor is a non-volatile compound, the reaction occurs in a 200 nm ring surrounding the collapsing bubble [13]. In this case, the sonochemical reaction occurs in the liquid phase and not inside the collapsing cavity. The products are sometimes nano-amorphous particles, and in other cases, nano-crystalline. This depends on the temperature in the fluid ring region where the reaction takes place. The temperature in this liquid ring is lower than that inside the collapsing bubble, but higher than the temperature of the bulk liquid. Suslick [11] has estimated the temperature in the ring region as around 1900 °C. In short, in almost all the sonochemical reactions leading to inorganic products, nano-materials have been obtained. They vary in size, shape, structure, and in their solid phase (amorphous or crystalline), but they were always of nanometer size. [12]. Cavitation being a nuclei dominated (statistical in nature) phenomenon, such variations are expected.

In hydrodynamic cavitation, nano-particles are generated through the creation and release of gas bubbles inside the sol–gel solution [14]. By rapidly pressurizing in a supercritical drying chamber and exposing it to the cavitational disturbance and high temperature heating, the sol–gel solution is rapidly mixed. The erupting hydrodynamically generated cavitating bubbles are responsible for the nucleation, the growth of the nano-particles, and also for their quenching to the bulk operating temperature. Particle size can be controlled by adjusting the pressure and the solution retention time in the cavitation chamber. Use of the hydrodynamic cavitation for the same purpose has also reported in some literature [15].

In the present study, attempts have been made to reduce the size of the rubber latex particles (Styrene Butadiene Rubber) present in the form of aqueous suspension with 275 μm particle initial size to the nano-scale. Acoustic as well as hydrodynamic cavitation methods have been used to meet the objective. The mechanism of cavitation and theory of size-reduction has been taken into consideration to obtain and explain the formation of the aqueous nano-suspension of the SBR.

Section snippets

Acoustic cavitation

The specifications of the equipments used are as follows:

Results

In the case of ultrasonic bath, there was absolutely no change in the size of rubber latex particles at all the solid concentration levels studied. The initial particle size of 275 μm remained unaffected even after 2 h of treatment in the sonication bath. The reason for this can be explained on the basis of energy dissipation levels. The suspension was kept in a beaker and the beaker was kept in the bath. Though the efficiency of the bath was 34.69%, only 3% of that energy was transferred to the

Discussion

In the hydrodynamic or acoustic cavitation set-up, there are two possible reasons for the observed size reduction. One of the possibilities is that, when a cavity collapse takes place, the shock wave generated travels through the liquid media generating local pressure gradient and fluid shear causing attrition of the solid particles and the reduction in the particle size. Other possibility is that when the cavity collapses, asymmetrically on the surface of the solid surface it produces a high

Conclusions

  • 1.

    The hydrodynamic cavitation has proved to be very effective in reducing the size of the elastic material like rubber efficiently. HC2 is more efficient compared to all the equipments tested in this work. Acoustic cavitation set-up can do the size reduction by adjusting the operating parameters such as increasing the power input per unit volume and/or irradiation intensity (W/m2) and decreasing the solid concentrations. The time of the operation varies depending on the final required size and

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