The formulated product to be measured is a skin cream (moisturiser). The moisturiser is a two phase system comprising of an oil phase and water phase. For this essay, I will consider the product as oil in water emulsion. Examples of such products found in the market today include: Olay Total Effects Facial Moisturiser, Nivea daily moisturiser and Johnson baby lotion. The reverse technology; water in oil emulsions are also available. A facial moisturiser is designed primarily to protect the skin from drying out.
Moisturizers do this by holding water in the stratum corneum, the outermost layer of the skin. They can also help protect your skin from the environment by creating a barrier on your skin that keeps oils from escaping and harmful outside elements from causing dryness or irritation. The consumer acceptance of a skin cream is very much dependent on its “feel” when taken from the jar or tube and applied to the skin. There are two key moments of truth which determine the overall consumer experience. The first is when the product is dispensed from the pack and as it rests on the hands.
The second is when the product is applied or rubbed onto the face. These ‘experiences’ are governed by its rheological properties. The shear thinning characteristic of skin creams is important to delivering the desired product performance. Rheology shear rate tests measure this behaviour. Rheology is the science dealing with flow behaviour and deformation of materials. Rheological instrument and measurements have become essential tools in analytical laboratories of companies for characterising ingredients and final products, as well as predicting product performance and consumer acceptance.
Knowledge of the rheological properties for these emulsions is important in the design and optimisation of processing equipment for production; checking product meets specification and quality control. Polymers and surfactants are included in the formulation to disrupt coalescence of the oil phase droplets. By studying the microstructure at low stresses, one can predict storage stability. Product development processes of facial moisturisers such as new ingredient selections, formulation preparations and material packaging are associated with a complex flow of materials.
Therefore, rheological measurements can assist researchers in both formulating products and improving processing efficiency. Rheological Measurement of Viscoelastic Emulsions In principle, rheological measurements determine the flow and deformation behaviour. The physical flow behaviour is then associated with the products internal structure. The operator loads the sample into the Rheometer and selects the appropriate experiment; the instrument carries out the test. Compatible software records the resultant data, graphs are plotted and tables are produced for analysis and comparison.
Specialised knowledge of the skin creams microstructure and a reproducible rheological test capable of decomposing the rheological behaviour into individual viscous and elastic components is necessary. Shearing of the sample is important to understanding its flow behaviour. A sheared flow can be achieved through flow between two parallel plates, rotational flow between coaxial cylinders or telescopic flow through capillaries. For the purpose of this essay I will consider the technique utilising stationary flow between two parallel plates on a Rotational Rheometer.
Rotational or Shear Rheometers control the applied shear stress and measure the resulting shear strain or apply a shear strain and measure the resulting shear stress. Figure 1 shows a schematic of components for a TA Instrument Rheometer. A rotating spindle is controlled by an electronic induction motor. The required geometry is fixed on the treaded end of the spindle. A digital encoder consisting of a light source and photocell read off the discs attached surrounding the spindle. Diffraction pattern are produced as the disc moves under an applied torque (Chhabra & Richardson 2004).
Figure 2 demonstrates the fluid deformation as a result of one rotating plate. This creates a laminar flow of layers which resembles the displacement of individual cards in a deck of cards. The parallel-plate model helps to define both shear stress and shear rate. Shear rate can be increased by either increasing rotation rate or decreasing gap between two plates. A force F applied tangentially to an area A being the interface between the upper plate and the liquid underneath, termed the “shear stress” ? (force/ cross sectional area) with units p. s, leads to a flow in the fluid layer.
The velocity of flow that can be maintained for a given force is controlled by the internal resistance of the liquid, i. e. by its viscosity. The shear stress ? causes the liquid to flow in a special pattern. A maximum flow speed Vmax is found at the upper boundary. The speed drops across the gap size (y1 from figure 1) down to Vmin = 0 at the lower boundary contacting the stationary plate. One laminar layer is then displaced with respect to the adjacent ones by a fraction of the total displacement encountered in the liquid between both plates. The speed drop across the gap size is termed “shear rate” with units 1/s.
The shear stress causes strain in solids but in liquids it causes the rate of strain. This means that solids are elastically deformed while liquids flow (Miller 2000). The correlation between shear stress and shear rate defining the flow behavior of a fluid is graphically displayed in a diagram of Shear Stress vs Shear rate. This diagram is called the Flow Curve. Viscosity Curve is another useful diagram. Viscosity (? ) is plotted versus the Shear Rate. This technique is useful in obtaining viscosity and stress data at high shear rates. The shear rate can easily be adjusted by manipulating the frequency or gap size.
Viscoelastic samples are easy to load and unload as compared versus Cone and Plate or Concentric Cylinder Rheometers. Three different types of tests can be conducted on a TA Instrument Rheometer: Steady Shear, Oscillatory and Creep tests. This makes the instrument cost effective. Errors in the data could be caused by inertia forces opposing flow. They tend to pull the plates together. Shear heating may occur at high shear rates and high product viscosity. A rise in the bulk sample temperature, leads to an increase in structural flow whilst lowering its viscosity.
This induces an error in final results. The operator must also be aware of errors due wall slippage and edge effects. By collecting data at different gap sizes, one can determine the extent of these effects (Vicente 2012). The Microstructure of a Skin Cream The visual and sensory properties are among the most important characteristics. Consumers have various expectations of moisturising emulsion, for instance, creaminess, body and consistency. These dictate the buying preference for consumer. Thus, understanding the Rheology properties is crucial to the market success of a product launch.
Figure 3 demonstrates a typical viscosity curve for typical shear thinning emulsion. Shear rates to the order for 10-6 represent the sample at rest under gravitational force. Here the emulsion behaves like a Newtonian fluid. The higher the viscosity at these lower shear rates, the more resistance to motion. This is due to a dense polymer network and finely broken oil droplets. A higher at rest viscosity gives the emulsion greater storage life stability. This at rest viscosity is experienced by the consumer when they make contact with cream in the jar and as it rests on their hand.
By taking the shear rate up an order of magnitude, the emulsion begins to flow with less resistance. This is termed shear thinning. This phenomenon is advantageous during processing because less energy is required when pouring, pumping and mixing. With increasing shear rates, matchstick-like particles suspended in the continuous phase will be turned lengthwise in the direction of the flow. Chain-type molecules in a melt or in a solution can disentangle, stretch and orient themselves parallel to the driving force. Particle or molecular alignments allow particles and molecules to slip past each other more easily.
Increasing the shear rate by another order of magnitude (for example rubbing onto the skin) causes the internal structure to realign in the direction of the flow. This lowers the viscosity at higher shear rates, until a maximum shear thinning viscosity is reached. Another viscosity plateau is achieved and product displays Newtonian behaviour. The shear thinning behaviour means that the product is easily spread across the skin when rubbed but does not pour off from the hands, delivering the desired consumer experience.
Since smaller particles are less deformable, hence smaller size oil droplets will increase viscosity. A decrease in the particle size leads to an increase in surface area. Therefore an increase in stabilizers may be required to avoid coalescence. Fatty Alcohols are included as a co-emulsifier adding rigidity/ structure to the emulsion droplet. Polymeric thickeners are water swellable that bind water and ‘thicken’ the aqueous phase. The choice and quantity are optimised to deliver the correct consumer feel. Particles