We stir up products, pumps, distribution, and pour the spray is usually thought of as a fluid. Surprisingly, however, it can be shown that they are living a double life calmly, and if fluid, but the retreat of the little-known soft-solid when left to their fate. Because of this double life imparts many desirable, processing, storage and performance attributes of manufactured products, it is important to understand it.
Example of viscosity measurement

An interesting paradox of viscosity measurement helps to illustrate this point. Consider the two skin treatments: foot cream, easy to apply and rub, silky finish, and diaper rash and eczema soothing emollient, known for its thick and heavy feel, when applied to the skin. Carried out to check the viscosity of the method is simple. Rotational benchtop viscometer equipped with T-bar spindle and Helipath stand (Brookfield Engineering, Middleboro, MA), which is well suited for soft solids, and run fast, single speed, the viscosity of each test to confirm the relative viscosity of the cream (see Figure 1).. The measurement results were unexpected: thin foot cream: 13,740 poise (or 1,374,000 CP), and thick emollient: 10,520 poise (or 1,052,000 CP). Viscometer has worked well, but the method is set to one of these products. How can we deliver a high-viscosity emollient cream feet lower than the measured viscosity value?

Figure 1 - Rotational viscometer with Helipath
stand and T-bar spindle.

Undisturbed for a closer inspection of the samples in their bathroom, a different picture appears: a low-viscosity foot cream really seems to be seen creamier emollient degree of robustness. In other words, the relative consistency of one or disseminate material or poking. This double life example of structured fluids.

Products, such as suspensions, emulsions and gels, with structure and internal colloidal interactions, polymer entanglements and associations to promote the product as a solid performance as the regular networks. These solid characteristics similar example can be seen clearly mayonnaise. Consider a large spoonful of mayonnaise on a plate. Does not seem to leak at all. In addition, if the mound is pushed gently with a spoon of mayonnaise, slightly deformed, but the rebound back to its original form, remove the spoon. This is due to elastic deformation due to stress mayonnaise and its subsequent restoration of the stress imposed on the removal. In other words, mayonnaise exhibits solid-like behavior. However, this type of behavior is apparent only when there is a gentle handling of mayonnaise. If you pushed a little harder to design "breaks" and the flow of mayonnaise, demonstrating the fluid-like behavior. If the stress is reduced, gel-like structure and the strength of contraction immediately returned.
Two-state solution

These two states of deformable soft and elastic solid points carefully and freely flowing liquid, formed at higher stresses drastically different. In addition, the "fluid," as we tend to label them, to spend most of their low-stress-rest of life is one of the containment integrity of the bottle or bath during storage on the shelf. Only a few moments, when they are pumped, spread, mixed, or sprayed, they experience life as a liquid, the liquid structured a double life. Soft solid structure these products should not be overlooked or ignored. Structure contributes to a number of critical performance attributes, such as:

* Texture, skin, oral and topical products and foodstuffs

* PPG and recession resistance

* Penetration

* Left to drain naturally and draining characteristics.

Thus, by measuring the structural properties of soft solid, such as stiffness and strength, is very valuable. Therein lies the problem: not suitable for measurement of particulate fluid measurement method (ie, viscosity). In addition, the measurement method of solid materials (ie, stiffness, strength, or hardness) is meaningless when applied fluid. The best way to deal with this just to do both the structure and flow studies, more meaningful, a detailed description of the product.
The flow test methods

Figure 2) The test method is called the shear rate a ramp. b) Viscosity profile of shea thinning material (pseudoplastic behavior). c) Viscosity Profile cutting thickening agents (dilatant
behavior).

Most companies perform a single point viscosity measurement quality control check. This means that a single meaning of the shaft speed and a number of reports, as explained above. Formulators will start the flow curves of viscosity during the test R & D to develop a product. It covers the different speed test track, as the shear viscosity of the exchange rate function. Figure 2 shows the test method and schedule of possible outcomes. Sometimes, QC, will be required to run more than one place at a test in order to ensure optimal product quality.

Spindle viscosity test selection is normally made for research and development of disc-type spindle is commonly used when investigating a large number of samples (not less than 0.5 L). When the sample is limited (<20 ml), coaxial cylinder geometry (cylindrical spindle inside a cylindrical chamber) option. Smaller sample volumes (<2 ml) and cone / plate geometry is an alternative. Figure 3 shows images of each type of spindle. There are a lot of other spindle geometry, such as T-bar has already been mentioned that certain types of materials.

Two sample materials described above were also tested with a cone / plate geometry. Shear rate chosen was 250 sec-1, which is associated with the application of skin creams constantly rubbing action. Test results were as follows: thin foot cream: 543 cP thick emollient: 3460 cP.

In Figure 3.) Disc spindle. b) Coaxial cylinder spindle geometry. c) Cone / plate
spindle geometry. d) the vane spindle.

Cone / plate method provides means to simulate the shear rate and rubbing test data confirms that, as expected, the thin legs cream exhibits much less than the thickness of the emollient, viscosity.
Structure of the test methods

For years, many viscometer users, in fact, knowing the structure of the tests carried out by a very good effect. T-Bar viscosity is a structural measure of strength. The main parameter is the torque of the T-bar spindle, driven by Helipath stand, cut path through the soft and hard sample. As such, it has more to do with the stress points that must be applied to disrupt the structure and initiate a high level of traffic than the true viscosity under shear yield. What other methods could be established to understand the structure of the material?
Material strength

Two common methods to measure the following values:

First Use of standard viscosity test, shear stress plot, compared with the shear rate curve and the extrapolation of the data stream to zero shear rate (see Figure 4).. This method provides a dynamic yield strength, as it concludes, after the liquid has been allowed to marry.

The second Use a tool to measure the static yield stress directly. This means that the material must be measured at the resting condition. Controlled stress rheometers this option (see Figure 4b).. Method is to apply a gradually increasing torque of the shaft and found the time axis starts to rotate.

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