Does Density of Microspheres Matter?
There are a lot of variables for the researcher to consider when selecting the right spherical particle for their research project. The selection of product is specific to the application, but to make the right determination it is important to know what properties of the particle are critical.
- Particle size and distribution?
- Material formulation?
- Color or fluorescence properties?
- Conductivity with metal coatings?
- Optical Properties? Reflective Index?
Typically, researchers have thought about most of these questions and a general idea of what particles they need to work with. Unfortunately they often misunderstand the critical role that density of microspheres plays in the success of the project.
When offering a product recommendation for spherical particles, such as microspheres, Cospheric technical support team usually starts by asking the researcher: “Does your project require specific density of microspheres?”
When is the density of microspheres important?
Density of microspheres controls the buoyancy of each particle and its behavior in a system or process. The density of microspheres becomes critical when specific buoyancy is desired for the application. Is it necessary for the microsphere to float, sink, or stay suspended in solution? If so, true particle density of microspheres is a critical parameter.
Let’s look at three scenarios:
#1 Suspension of microspheres in a specific fluid
Often the scientists are looking for spherical particles that will have a specific behavior in solution (e.g. stay suspended, float to the surface, or sink to the bottom). In this situation the delta between the densities of the particle and of the fluid need to be precisely controlled. If there is any mismatch in density of microspheres and the density of fluid the particles will either sink or float.
According to Stoke’s Law, density delta, or the difference in the density of the particle and the density of the liquid, is one of the most critical variables in determining settling velocity of the particle. Settling time and velocity are proportional to density delta (also known as density mismatch).
The formula for the Settling velocity vT is given by:
- vT is the terminal velocity of a spherical particle
- g is the gravitational acceleration – for Earth, equal to 9.8m/s2
- d is the particle diameter
- ρp is the true density of the particle;
- ρm is the density of the fluid; and
- μ is the dynamic viscosity of the fluid.
Obviously, we need the particle to be heavier than the fluid in order for it to sink and the particle to be lighter than the fluid for it to flow to the surface. The larger the difference between the densities the easier the separation.
The situation becomes challenging when it is desirable that the particles remain suspended in the fluid for a significant period of time. In this case, we would want the true density of microspheres and the density of the fluid to be as close as possible, thereby slowing down the settling and maximizing the time in suspension.
Matching the density of the particle to the density of the liquid is very important for minimizing settling velocity and maximizing the time microspheres spend in suspension.
Matching the density of the fluid and the density of microspheres precisely is not an easy task. Keep in mind that even the slightest variation in density matters, and densities of liquids sometimes vary significantly with changes in temperature, pressure, and materials added to it. Please see Density of Aqueous Solutions Reference Tables.
If the experiment requires that larger microspheres (such as close to 1mm in diameter) stay suspended for some time, the density delta needs to be at most 0.001g/cc. It is very difficult, if not impossible, to control the density of each individual particle that precisely. Most accurate density equipment that is commercially available cannot even produce reliable density measurements to that tolerance. Moreover, even slight changes in the temperature of the fluid might change its density and determine whether the particles will stay suspended.
According to Stoke’s Law, settling velocity is proportional to the square of the diameter of the spheres. That means that smaller microspheres will sink at a significantly slower rate and are much less sensitive to the density mismatch. When long-term suspension of particles is desired, the move to smaller sized particles may need to be considered to maximize the settling time, while being able to hold realistic density tolerances.
It is always advantageous to use the highest quality precision density particles available on the market.
#2 Fluid flow, performing flow visualizations, or Particle Image Velocimetry
Particle Image Velocimetry (PIV) and Flow Visualization are quantitative imaging techniques that use the movement of tracer particles to observe, record, and analyze the behavior and movement of fluids that would otherwise not be visible to us. In these techniques the base fluid is seeded with highly visible tracer particles that will ideally stay suspended in the base fluid for a significant period of time and move with the fluid.
These methodologies are highly dependent on the selection of the suitable tracer particles and the ability of these particles to reliably trace the flow.
As we discussed above, in order to achieve that long term suspension of tracer particles in the suspending medium, the density delta between the particle and the fluid needs to be minimized as much as possible. Precisely controlled density of each individual particle is paramount to the successful implementation of the technique.
In this scenario, in addition to the critical density requirement, there is extra complexity of spherical tracer particles needing to be clearly visible as they follow the flow of the fluid.
In these applications, not only does the density of microspheres need to match the density of the fluid exactly, the particle needs to also be small enough to stay suspended, large enough to be visible, be color-stable, and visible to the observer under the specified illumination (laser, UV light, daylight, etc) during flow visualization or particle image velocimetry process.
An example of tracer particles widely used in PIV applications is sliver-coated hollow-glass microspheres. The advantage of these seed particles is that they are opaque and highly reflective to any light including laser. The density delta in aqueous solution is acceptable for smaller sized particles.
#3 Simulation or modeling of objects with specific density
Whether studying the flow of fish eggs in a stream, the spread of pollen in forest, or the flow of semen in artificial insemination, many experiments start with a computer model that is based on an assumption that the model particle is a sphere of specific density.
For example, when studying environmental toxicity of microplastics, a researcher should note that differences in shape and density cause microplastics to disperse diversely in different compartments of the aquatic environment (water surface, water column and sediment) and influence their availability to organisms at different trophic levels and/or occupying different habitats. Taking this into account, in order to get comparable data, the researcher should choose test particles to be as close in shape and density to the actual type of microplastic they are studying.
Scientists who study fish require artificial micro-particles to simulate fish eggs and their dispersion behavior in water. In order to accurately simulate the dispersion of fish eggs, it is important to ensure the proper size and density of microspheres, such as sea water density particles of 1.025g/cc (UVPMS-BG-1.025), and fresh water density beads of 1.00g/cc.
Being able to obtain such particles to test and validate the model is often a key step towards understanding the process and making breakthroughs in understanding how our world functions. The size, the shape, and the density of microspheres will be critical to testing the accuracy of the model.
Bulk Density vs. True Particle Density:
When selecting particles by density it is important to differentiate between bulk density and true particle density.
Particles are often sold as dry powder. Some companies that sell particles in large volumes for applications that do not require a lot of precision might list bulk density of the powder. This is misleading. Bulk density, also known as volumetric density, refers to the total volume that a mass of material occupies.
Bulk density is not an intrinsic property of a material and it can change depending on how the material is handled. How well is the powder packed? Is it compacted or fluffy? How was the material poured? Was it tapped before the volume got measured?
The precision density of microspheres, which is referred to in Stoke’s Law and the microsphere property that we have been referring to in the three scenarios described above, is true particle density. It is the density of the individual particles that make up the powder. Even though this parameter is very challenging to measure precisely for very small particles and requires specialized equipment and methodology, it is an inherent property of material and is a function of exact composition of the raw materials that the particle consists of.
When selecting materials of precise density for your next research and development project, look for precision in true particle density.
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