*2.1.1 High pressure valve homogenizer (HPVH)*

*Nanoemulsions - Properties, Fabrications and Applications*

used are the ongoing trends in nanoemulsions.

**2.1 High energy methods for emulsion formation**

**2. Fabrication**

widely used methodologies.

and low energy methods, described in the next section) many steps were involved, a single step method was developed using vapor condensation. Moreover the traditional methods used in the preparation of Pickering nanoemulsions had some disadvantages. High energy methods reduced the adsorption of the particles on the droplets while, low energy methods were unable to produce Pickering nanoemulsions and clogging of nanoparticles was seen. The concentration of nanoparticles required in the new methods was also less compared to that required in traditional methods. In this process, oil was taken and cooled below the dew point, during which the water condenses on the oil. If the oil has the right properties and sufficient concentration of nanoparticles, then water drops self-disperse within the oil. The nanoparticles then will self-assemble around them to form nanoemulsions [15]. Nanoemulsions are studied in great detail due to their potential applications. Improvements in their preparation methods and the fields in which they can be

The fabrication of nanoemulsion involves the preparation of macroemulsions and then it's conversion to nanoemulsion by various methods, all of which can be categorized into either Low energy or High energy methods [5, 16]. Techniques which involve modification of factors responsible for the hydrophilic–lipophilic balance come under Low energy methods and those that use mechanical devices to break down the particles to small sizes are referred to as high energy methods. As much as composition is responsible for the properties of the nanoemulsion so is the technique used for its preparation. In this section a brief insight is given on a few

In contrast to the low energy methods for nanoemulsion formation, high energy methods require the use of many devices which uses mechanical or

chemical energy as input to form small droplets because they are non-equilibrium systems which cannot be formed spontaneously [17]. These devices often entail a huge initial cost as well as expenses to maintain throughout use. The purpose of these devices in high energy methods is to provide intense mechanical energy that helps to break up macroscopic phases or turn larger droplets into smaller droplets [18]. These devices provide forces so strong that it disrupt water and oil phases to form nanoemulsions. In high energy methods, input energy density is about 108–1000 W/kg. The required energy supplied is in very shortest duration of time to the system in order to obtain homogeneous small sized particles. In addition to this, the high energy methods for 55 nanoemulsion formation are not limited by the types of oil and emulsifiers that can be used like the low energy methods are. At present high energy methods are more frequently utilized in the food industry than low energy methods with high pressure valve homogenization, microfluidization, and sonication being the most common [19]. All this high energy methods are impacted by emulsion component characteristics (i.e. oil, type, surfactant type, surfactant concentration, viscosity, etc.) and equipment characteristics (i.e. size of the equipment, pressure used, number of passes/time in equipment, design, etc.). The input energy density is about 108–1010 W/kg [20]. These parameters should be optimized for each and every system and high

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energy method.

HPVH is the most popular method used for the production of nanoemulsions. The most common use is in applications from ketchup processing to milk homogenization and to manufacture nanoemulsions that particle sizes are up to 1 nm [18]. When using a HPVH, a coarse emulsion is initially made using a high-speed mixer, fed into the input valve of the HPVH, and then flowed between the valve seat and valve at a high velocity. The macroemulsion is forced to pass through a small orifice at an operating pressure between 500 and 5000 Psi [18]. Since several forces like hydraulic shear, intense turbulence and cavitation act together extremely small droplet sized nanoemulsions are achieved. The process is repeated till the final product reaches our desired droplet size and polydispersity index (PDI). Lower the PDI means higher uniformity of droplet size in nanoemulsions. Mono-disperse samples have PDI lesser than 0.08, narrow size distribution range is 0.08–0.3 and PDI greater than 0.3 indicates broad size distribution [18].

With an increase in velocity, the pressure decreases causing an instantaneous pressure drop and encouraging the coarse emulsion to impinge on the impact ring [21]. Sometimes HPVH passes through two valves and thus emulsion production will break up into two stages: in the first stage the droplets are broken up and in the second stage a lower pressure is utilized to disrupt any 'flocs' formed by the initial valve [22]. Obtaining submicron levels requires large amount of energy and high temperature which can deteriorate the components. Thermolabile compounds like proteins, enzymes and nucleic acids may be damaged easily [18] (**Figure 1**).

**Figure 1.** *Schematic representation of high pressure valve homogenizer [18].*
