Demystifying Plastic Flow : Non-Newtonian Pharmaceuticals

Ever wonder why ketchup flows smoothly after a good shake, but honey seems to defy gravity? The answer lies in the science of non-Newtonian fluids like plastic flow, and it turns out, many medications behave in this unexpected way too!

Beyond Simple Liquids

Most of us think of liquids as flowing freely, regardless of pressure. Water is a perfect example. But many pharmaceutical products, from syrups to creams, are different. They belong to a category called non-Newtonian fluids, meaning their flow behavior changes with applied force.

Imagine toothpaste. You need to apply some pressure before it starts coming out of the tube. This is because toothpaste is a suspension, a mixture of solid particles (like silica) dispersed in a liquid (like glycerin). The way these particles interact determines how the toothpaste flows.

Understanding Flow: Three Key Players

Scientists use a special instrument called a viscometer to measure a fluid's resistance to flow, known as viscosity. When it comes to non-Newtonian fluids, things get interesting. There are three main types of flow behavior:

• Plastic: Similar to toothpaste, plastic fluids require a certain amount of force to overcome a resistance point before they start to flow. This "yield value" determines how much pressure is needed for the medication to come out of the container.
• Pseudoplastic: This is the most common type for pharmaceuticals. Think ketchup! As you apply force (shaking the bottle), the viscosity decreases, and it flows more easily. This allows for easier pouring or swallowing.
• Dilatant: This is the opposite of pseudoplastic. Here, the viscosity increases with applied force. Cornstarch mixed with water is a classic example. The more you stir it, the thicker it becomes! Thankfully, this behavior is less common in medications.

Why Does This Matter?

Understanding how medications behave as non-Newtonian fluids is crucial for several reasons:

• Drug Delivery: The flow properties can impact how effectively a drug is absorbed by the body.
• Dosing Accuracy: Measuring precise doses, especially with creams or gels, can be challenging with non-Newtonian behavior.
• Patient Experience: Difficulty squeezing medication out of a tube or swallowing a thick syrup can affect how well patients follow their treatment plan.

By considering these factors, scientists can formulate medications with optimal flow characteristics. This ensures efficient drug delivery and a positive experience for patients.

So, the next time you reach for your medicine, remember, the science behind its flow behavior might be more fascinating than you think!

Plastic Flow

Have you ever wondered why toothpaste needs a squeeze to come out of the tube, while honey seems to defy gravity and flow slowly? The answer lies in a fascinating scientific concept called plastic flow. This blog post dives into the world of plastic flow, exploring its properties and how it impacts various pharmaceutical products.

Bingham Bodies and the Yield Value of Plastic Flow

Plastic flow is a type of non-Newtonian flow behavior exhibited by certain materials. Unlike water, which flows consistently regardless of pressure, these materials require a minimum amount of force, known as the yield value, to start flowing. Imagine ketchup – you need to shake it (apply force) to overcome its initial resistance before it starts pouring smoothly.

Substances that exhibit plastic flow are called Bingham bodies, named after Eugene Bingham, a pioneer in the field of rheology (the science of flow). The yield value is a crucial property as it indicates the strength of interactions between particles within the material. In pharmaceutical products like toothpaste, these interactions can involve suspended particles and the surrounding liquid. A higher yield value suggests stronger interactions, requiring more force to initiate flow.

Understanding Plastic Flow Behavior

The behavior of plastic flow can be visualized using a graph called a consistency curve/ Rheogram. This curve plots the relationship between shear stress (force applied) and rate of shear (how fast the material flows). In plastic flow, the curve doesn't start at zero. Instead, it intersects the shear stress axis at the yield value. Once this yield value is exceeded, the material starts flowing, and the relationship between shear stress and rate of shear becomes more linear, resembling Newtonian flow (constant viscosity).

The Connection to Suspensions

Plastic flow is often associated with concentrated suspensions, where solid particles are dispersed in a liquid. These particles can form clusters or "flocs" due to weak forces like van der Waals forces. The yield value arises from the need to break these flocs before the material can flow. The more flocculated the suspension, the higher the yield value.

Other Details of Plastic Flow

Plastic flow is linked to the presence of closely packed particles in dense suspensions. These particles form a continuous structure within the substance. The reason behind the existence of a yield value, a minimum stress needed to start the flow, lies in the interactions between adjacent particles, primarily caused by van der Waals forces. These interactions need to be overcome for the substance to start flowing. Thus, the yield value serves as an indicator of the strength of these particle interactions.

The more packed the suspension is, the higher the yield value tends to be. Additionally, friction between moving particles can also contribute to the yield value. Once the yield value is surpassed, any further increase in shearing stress leads to a proportional rise in the rate of shear. Essentially, when stress exceeds the yield value, the substance behaves similarly to a Newtonian fluid, flowing more freely.

For example, let's consider a plastic material with a yield value of 5200 dynes/cm2. At shearing stresses exceeding this value, the stress "F" increases linearly with the rate of shear "G". If the rate of shear was 150 sec-1 when the stress reached 8000 dynes/cm2, we can calculate the plastic viscosity "U" using the given equation. By substituting the values into the equation, we can find the plastic viscosity of the sample. This calculation helps in understanding the flow properties of the material under specific conditions, aiding in its formulation and processing.

Understanding these principles of plastic flow is essential for designing pharmaceutical formulations and ensuring their proper functionality during manufacturing processes. By comprehending how substances behave under stress, researchers can optimize formulations to meet desired performance criteria and ensure product quality and consistency.