Particle shape needle-shaped

Particle shapes are classified as acicular. A, monoclinic and blocky, M, or monoclinic and needle, N. 

Particle shape, as indicated in figure under 100X magnification shall be free from hair shape or needle shape particles... 

Solids appear in one of two forms, either as crystals or powders. The difference is one of size, since many of the powders we use are in reality very fine crystals. This, of course, depends upon the manner in which the solid is prepared. Nevertheless, most solids that we encounter in the real world are in the form of powders. That is, they are in the form of discrete small particles of varying size. Each particle has its own unique diameter and size. Additionally, their physical proportions can vary in shape from spheres to needles. For a given powder, aU grains will be the same shape, but the particle shape and size can be eiltered by the method used to create them in the first place. Methods of particle formation include ... 

Particle shape. When examined under the micro scope the fracture of the particles shall be con choidal and there shall be no long needles or round particles... 

Nonspherical particles. Little is known about the fall velocity of nonspherical particles. It would be expected that needle-shaped particles would feel less air resistance per unit mass and fall faster but that irregularly shaped particles would, in general, feel more resistance and fall more slowly. The net effect depends upon the distribution of particle shapes, which is also uncertain. 

Dry-powder inhalers (DPIs) deliver the drug to the respiratory tract in aerosol form. An aerosol is by definition a suspension of free liquid or solid fine particles in a gas phase, which is air in the case of DPIs (and a compressed gas in the case of needle-free injection). The most prominent characteristic that determines the delivery of drug particles to the lungs is the particle size, although particle shape and density are also of considerable importance for the behavior of an aerosol in the respiratory tract (Brain and Blanchard 1993 Gonda 1992 Heyder et al. 1986 Agnew 1984 Heyder et al. 1980). 

Besides particle size and dmg concentration, particle shape and density of powders are also important factors for achieving a homogenous mixture. Powders with nearly spherical particles are easier to mix than those with irregularly shaped particles. Micronized, needle, or flat particles require longer blending time due to aggregate... 

Table 2.4 Qualitative terms for particle shape Acicular needle-shaped...

Three particle systems were examined by image analysis and their aspect ratio and particle size distributions were measured [197,198]. The data were then used as a reference method for neural networking using a Malvern Mastersizer X and concentrations from 2 g f to 60 g I" . Particle shapes ranged from an ellipsoidal cracking catalyst, needle shaped asbestos and monoclinic sucrose crystals. 

Besides the numerical calculation of shape factors, there are also qualitative descriptions of shape. For example, certain pharmacopoeias define shapes such as an agglomerate, needle, etc. Since no one method is best in all applications, the best method for determining particle shape depends on the application and nature of the particle being examined. 

Particle shape can also affect blending processes. Spherical and cubic shaped particles typically exhibit good flow properties and therefore promote blending. However, readily flowing materials may also be more prone to segregation. Plates and needle shaped particles have poor flow properties, are harder to dilate/expand, and are more likely to agglomerate. As a result, it may be more difficult to achieve uniformity when blending plate and needle shaped particles. Conversely, a benefit to this decreased mobility is that once blended, these are more likely to stay blended. 

Consider two limiting anisotropie particle shapes platelet-like and needle-like. The platelets, when packed in a flat sample holder, will tend to align parallel to one another and to the sample surface. Then, the amount of plates that are parallel or nearly parallel to the surface will be much greater than the amount of platelets that are perpendicular or nearly perpendicular to the surface. In this case, a specific direction that is perpendicular to the flat... 

Figure 3.20. The two critical cases of non-random particle orientation distributions due to distinctly anisotropic particle shapes platelet-like (left) and needle-like (right) particles. The arrows indicate the directions around which the particles may rotate freely.

Particle shape irregular, needle Crystal structure monoclinic... 

The harder the crystal, the more brittle and easier to break. Particle shape will also play a part, i.e., spherical crystals don t break easily, needles do, etc. 

Solids concentrations can vary from a few percent to well over 50% in a typical stirred tank. Solids concentration, particle shape, and the viscosity of the suspending phase are the main factors affecting the rheology and settling characteristics of the slurry. Cubic- and spherical-shaped solids tend to form Newtonian slurries, while needle-, oblong-, and plate-shaped solids form thixotropic slurries. Such slurries exhibit yield stresses even at quite low solids concentrations. This can lead to the development of caverns, as shown in Section 9.4. Proper design can usually overcome these stagnation problems. 

At this point, we are most interested in the size of particles and how the other factors relate to the problem of size. However, particle shape will determine how we define size. Most of the particles that we will encounter are spheroidal hi shape. But. if we discover that we have needle-like (acicular) particles, how do we define their average diameter Is it an average of the sum of length plus cross-section, or what ... 

The viscosity of slurries is a function of the solution and solid involved, as well as the slurry density. The viscosity can also be significantly affected by the particle size, size distribution, and particle shape. As a general rule, as particle shape varies from spheres to needles, the viscosity moves further from Newtonian behavior. A detailed discussion of factors affecting the viscosity of suspensions can be found in Sherman (1970). 

Before choosing a particle sizing technique, examination of the samples under a microscope is usually wise becau.se the range of sizes and shapes present can then he estimated. Most particle size methods are sensitive to particle shape and all are limited with respect to the particle size range. Particles with approximately spherical shapes are measured most accurately. Needles and other shapes that differ significantly from spherical are often analyzed by microscopy. 

---