Computational Fluid Dynamics Blogs

Exquisite events and parties with enormous amounts of food and drinks need a showy piece which could be a chocolate fountain or a cheese wheel. These chocolate fondue fountains are strikingly appealing to a universal crowd, and for those on a low-sugar diet, dip your fruits and rejoice in a guilt-free pleasure. Beyond the beauty of that dripping chocolate, ever wondered about the physics behind the fountain? Let’s dive deep into the history and fluid dynamics of this non-Newtonian fluid fountain.

History of the Chocolate Fountain

The chocolate fountain was invented by Ben Brisman and was introduced in the early ’90s by a Canadian company, Design & Realisation (D&R). These fountains weren’t so popular during their early years and, for better visibility, were displayed at the National Restaurant Show in Chicago. In 2001, Jay Harlan, a former Mariott executive’s company, Buffet Enhancements International, partnered with D&R to distribute the chocolate fountain to various hotels and resorts in the US. Two years later, Sephra in California stepped into the market and started selling high-end chocolate fountains that were manufactured in China. They improved on the existing designs with easier operation and simplified cleaning mechanisms. To accommodate the varying client needs, different size options were introduced. These new designs were followed by heavy advertisements, and the result was the growing popularity of these fountains for catering services worldwide.

Chocolate: A Non-Newtonian Fluid

The fluid dynamics of liquid chocolate in a fondue fountain are a lot different from that of a water fountain. This is because chocolate is a non-Newtonian fluid. Now, what is a non-Newtonian fluid? Well, non-Newtonian fluids are those that do not follow Newton’s law of viscosity. In these fluids, the viscosity can change to be more liquid-like or more solid-like, depending on the force applied. A good example would be ketchup in a bottle, which has a runnier consistency when the bottle is shaken, and hence it is a non-Newtonian fluid. The viscosity of non-Newtonian fluids largely depends on the shear rate but can also display a time-dependent viscosity, unlike Newtonian fluids, where the shear stress is directly proportional to the stress rate.

 The behavior of non-Newtonian fluids can be classified into four different categories and they are: -

Dilatant: They are also referred to as shear thickening fluid and their viscosity increases with the growing shear rate. A suspension of cornflour and water when compressed quickly by hand will almost turn solid and is an example of a dilatant fluid.

Pseudoplastic: These types of non-Newtonian fluids become less viscous with an increasing shear rate and are the opposite of dilatant fluids. Ketchup and chocolate are examples of shear-thinning fluid.

Rheopectic: They are similar to dilatant fluids, i.e., the viscosity increases with shear rate, and the difference between the both is that in rheopectic fluids, the increase in viscosity is time-dependent. Whipped cream is an example of rheopectic fluid because it gets thicker or viscous when whipped over some time.

Thixotropic: The viscosity of these fluids decreases with an increasing shear rate and is time-dependent. Yogurt is an example of a thixotropic fluid; it turns liquid-like when agitated for some time.

Fluid Dynamics of Chocolate Fountain

Understanding the fluid dynamics of non-Newtonian fluids like chocolate can be a challenging task, considering their changing behavior with time and under the application of force. A team of scientists at the University College London (UCL) undertook experiments to study why molten chocolate sloped inward and did not splash straight down, explaining the complex physics behind the working of a chocolate fountain. There are three separate profiles for fluid mechanics in the chocolate fountain, and they are –

Pipe Flow: The flow of molten chocolate in a basic tabletop chocolate fountain is operated by the rotation of a helical screw or by exerting pressure at the base of the pipe. The flow through the pipe is unidirectional and is a plugged flow using the power law. On the other hand, for Newtonian fluids, the flow is expected to be parabolic.

Dome Flow: This is the second flow profile of the chocolate fountain, and in this region, layers of molten chocolate are flowing over a plastic dome (with a small aspect ratio), creating a thin film flow. Beyond the slope, the dome shape is not important as the chocolate thins as it flows because the gravitational and viscous forces balance out.

Curtain Flow: As the chocolate is falling downward, surface tension helps in determining how far the sheets will be pulled inwards. An inviscid model cannot predict this inward pull to the extent a viscid model would.

The fluid flow profiles of a chocolate fountain provide more insights into the fluid mechanics of non-Newtonian fluids. From the first profile, i.e., the pipe flow, it is possible to develop a fluid flow theory of non-Newtonian fluids. In the second profile, the thin film flow provides an understanding of the lubrication theory by using scaling methods. The profile of the falling liquid chocolate is yet to be intervened and provides opportunities for further experimentation and study.

Studying the physics of a chocolate fountain is as relishing as the taste of a marshmallow dipped in chocolate. This study paves the path for physicists and engineers to learn more about the characteristic behavior and possible applications of non-Newtonian fluids. Further, these applications can be tested using CFD solutions, and Cadence Fidelity CFD tools have a lot to offer for unique phenomena testing and for understanding shear stress in fluids.

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