Internal droplets

Emulsion A dosage form consisting of a two-phase system comprised of at least two immiscible liquids, one of which is dispersed as droplets (internal or dispersed phase) within the other liquid (external or continuous phase), generally stabilized with one or more emulsifying agents. (Note Emulsion is used as a dosage form term unless a more specific term is applicable, e.g. cream, lotion, ointment.). 

Figure 22 Drug-release profile of isoniazid from multiple emulsion based on imi/oligo/droplet internal phase. (From Ref 57.)...

J. Bellan and K. Harstad. The details of cmivective evaporation of dense and dilute elusters of droplets. International Journal of Heat and Mass Transfer, 30 1083,1987. 

Hindmarsh, J.P., A.B. Russell, and X.D. Chen. Experimental and numerical analysis of the temperature transitirai of a suspended fieezing wato droplet International Journal of Heat and Mass Transfer, 46 1199—1213, 2003. 

Nikolopoulos, N., Strotos, G., Nikas, K. S., Bergeles, G. (2012). The effect of webernumber on the central binary collision outcome between unequal-sized droplets. International Journal of Heat and Mass Tranter, 55(7-8), 2137-2150. 

Gopireddy, S., Gutheil, E. (2013). Numerical simulation of evaporation and drying of a bi-component droplet. International Journal of Heat and Mass Transfer, 66, 404-411. 

In the past decade microscopic detection of specific types of coalescence within double-emulsion droplets has contributed to a better understanding of these mechanisms in order to control their occurrence. Three types of coalescence events have been identified in double-emulsion systems. These include coalescence between contacting inner droplets (internal coalescence), coalescence of interior droplets with the outer continuous phase (external coalescence), and coalescence between contacting two-phase emulsion globules. Direct evidence of each type of coalescence has been collected for double emulsions with microscopic visualization. The type and concentrations of surfactants in each phase of the water/oil/water double emulsions are critical in controlling the occurrence and type of coalescence (Ficheux et al., 1998 Villa et al., 2003 Hou and Papadopoulos, 1996 Hou and Papadopoulos, 1997 Pays et al., 2001). 

Figure 6.9 W/O/W emulsion prepared by SPG membrane emulsification for trans-catheter arterial injection chemotherapy of hepatocellular carcinoma (HCC) (a) Microscopic view of multiple emulsion droplets. Internal water droplets containing anticancer drug are visible as black dots, (b) Particle size distribution of oil droplets immediately after preparation and 40 days after preparation (Nakashima et al., 2000 Higashi et al., 1995).

Impingement demister systems are designed to intercept liquid particles before the gas outlet. They are usually constructed from wire mesh or metal plates and liquid droplets impinge on the internal surfaces of the mist mats or plate labyrinth as the gas weaves through the system. The intercepted droplets coalesce and move downward under gravity into the liquid phase. The plate type devices or vane packs are used where the inlet stream is dirty as they are much less vulnerable to clogging than the mist mat. 

Fig. 2. Aerosol emulsion droplets containing propellant (a) in the internal phase with subsequent formation of aerosol foam and (b) in the external phase...

Foam Inhibitors. Methyl sihcone polymers of 300-1000 mm /s(= cSt)) at 40°C are effective additives at only 3—150 ppm for defoaming oils in internal combustion engines, turbines, gears, and aircraft appHcations. Without these additives, severe churning and mixing of oil with air may sometimes cause foam to overflow from the lubrication system or interfere with normal oil circulation. Because sihcone oil is not completely soluble in oil, it forms a dispersion of minute droplets of low surface tension that aid in breaking foam bubbles. 

Internal Flow. Depending on the atomizer type and operating conditions, the internal fluid flow can involve compHcated phenomena such as flow separation, boundary layer growth, cavitation, turbulence, vortex formation, and two-phase flow. The internal flow regime is often considered one of the most important stages of Hquid a tomiza tion because it determines the initial Hquid disturbances and conditions that affect the subsequent Hquid breakup and droplet dispersion. 

The generated water vapor rises through a screen (demister) placed to remove entrained saline water droplets. Rising further, it then condenses on the condenser tube bank, and internal heat recovery is achieved by transferring its heat of condensation to the seawater feed that is thus being preheated. This internal heat recovery is another of the primary advantages of the MSF process. The energy performance of distillation plants is often evaluated by the performance ratio, PR, typically defined as... 

Emulsification is essential for the development of all types of skin- and hair-care preparations and a variety of makeup products. Emulsions (qv) are fine dispersions of one Hquid or semisoHd ia a second Hquid (the contiauous phase) with which the first substance is not miscible. Generally, one of the phases is water and the other phase is an oily substance oil-ia-water emulsions are identified as o/w water-ia-oil emulsions as w/o. When oil and water are mixed by shaking or stirring ia the absence of a surface-active agent, the two phases separate rapidly to minimize the iaterfacial energy. Maintenance of the dispersion of small droplets of the internal phase, a requirement for emulsification, is practical only by including at least one surface-active emulsifier ia the oil-and-water blend. 

Holdup and Flooding At this point it is useful to introduce the concepts of holdup and flooding in column contactors. It is normal practice to select the phase which preferentially wets the internals of the column as the continuous phase. This then allows the dispersed phase to exist as discrete droplets within the column. If the dispersed phase were to preferentially wet the internals, this could cause the dispersion to prematurely coalesce and pass through the column as rivulets or streams which would decrease interfacial area and therefore column efficiency. 

Satisfactory performance is obtained with tubes having helical ribs on the inside surface, which generate a swirling flow. The resulting centrifugal action forces the water droplets toward the inner tube surface and prevents the formation of a steam film. The internally rifled tube maintains nucleate boiling at much higher steam temperature and pressure and with much lower mass velocities than those needed in smooth tubes. In modern practice, the most important criterion in drum boilers is the prevention of conditions that lead to DNB. 

Spray Towers A spray tower consists of an empty shell into the top of which the liquid is sprayed by means of nozzles of various kinds the droplets thus formed are then allowed to fall to the bottom of the tower through a stream of gas flowing upwards. The use of sprays appears to offer an easy way of greatly increasing the surface area exposed to the gas, but the effectiveness of the m.ethod depends on the production of fine droplets. These are difficult to produce and suffer from the disadvantage that they are liable to entrainment by the gas even at low gas velocities. The surface area may also be reduced as a result of the coalescence of the droplets first formed. As a consequence of these effects, the large increase in surface area expected may not be achieved, or if achieved m.ay be accompanied by serious entrainment and internal circulation of the liquid so that true counter-current flow is not obtained. A single spray tower is suitable for easy absorption duties. For difficult duties, a number of towers in series can be used. 

Partieulate produets, sueh as those from eomminution, erystallization, preeipi-tation ete., are distinguished by distributions of the state eharaeteristies of the system, whieh are not only funetion of time and spaee but also some properties of states themselves known as internal variables. Internal variables eould inelude size and shape if partieles are formed or diameter for liquid droplets. The mathematieal deseription eneompassing internal eo-ordinate inevitably results in an integro-partial differential equation ealled the population balanee whieh has to be solved along with mass and energy balanees to deseribe sueh proeesses. 

Thompson, P.D., 1968. A transfomiation of the stochastic equation for droplet coalescence. In Proceedings of the international conference on cloud physics, Toronto, Canada, pp. 1115-1126. 

The basic purpose of an oil separator is to clean the pressurized air of any oil contamination, which is highly detrimental to pneumatically controlled instrumentation. A separator consists of an inlet, a series of internal baffle plates, a wire mesh screen, a sump, and an outlet. The pressurized air enters the separator and immediately passes through the baffle plates. As the air impinges on the baffle plates it is forced into making sharp directional changes as it passes through each baffle section. As a result, the oil droplets separate from the air and collect on the baffles before dropping into the separator s sump. 

---