Certification of 10 m Diameter Polystryrene Spheres ( “ Space Beads ” )

There are numerous examples in trade and commerce in which small particles play an important role, either as the commodity of trade itself, as unwanted contaminants in a product or an environment, or as a basis for comparison between a “normal” particle and an “abnormal” one. Many of the products we buy come in the form of small particles, powders, or particle suspensions, including medicines, cosmetics, food products, paints, talcum powder, cements, photocopier toners, and milk (which is essentially a suspension of microdroplets of fat in water). In other cases, small particles are unwanted, for example in cleanrooms for microelectronics or pharmaceutical manufacturing, in lubricating oils in motor vehicles, and in the air we breathe and the water we drink. There are also situations where one would like to compare a known, standard particle to an unknown, test particle as in the case of blood-cell testing. In all of these instances, particle standards play a key role, enabling quality control, product uniformity, conformance to standards, traceability to NIST, interchangeability of instrumentation, uniformity of measurement, or some combination of these goals. With these benefits in mind, the National Bureau of Standards set out, in the early 1980’s, to develop a range of particle-sizing Standard Reference Materials (SRMs) for use in calibrating and certifying instruments that measure particle size, whether as products, by-products, or contaminants. In cooperation with ASTM Committee E-29 on Particle Size Measurement, researchers in the Precision Engineering Division (PED) at NBS developed a series of five SRM’s consisting of monosized polystyrene microspheres with diameters of 0.3, 1, 3, 10, and 30 m [1-4]. The three smallest particles, donated by commercial vendors, were certified first, since such small microspheres were comparatively easy to grow using conventional polymer emulsion techniques. However, for the two larger diameters, 10 m and 30 m, the techniques that existed at the time did not yield rigid particles of the required sphericity because of the detrimental effects of gravity on the microsphere growth process. (At the time, there were techniques to make large microspheres, but these were relatively soft and unsuitable for use as SRMs, which must be rigid and stable.) At about the same time that NBS was certifying its series of particle-sizing SRMs, a group of researchers led by John Vanderhoff of Lehigh University and Dale Kornfeld of NASA Marshall Space Flight Center was conducting experiments aboard the space shuttle to determine the effect of microgravity on chemical reaction rates of emulsion polymerization, as well as on the morphology (shape) of polymer microspheres grown in microgravity [5]. The first experiments of the Monodisperse Latex Reactor (MLR) were conducted in 1982 aboard space shuttle flight STS-3 and resulted in microspheres as large as 5 m in mean diameter. A subsequent experiment on a later shuttle flight, STS-6, produced particles of 10 m mean diameter. In 1984, the NBS group obtained samples of the 5 m and the 10 m materials and did a detailed intercomparison between the space-made particles and earth-made particles of similar composition [1,2]. In both cases, the space-made materials were found to be superior in terms of individual particle sphericity, narrowness of size distribution, and, importantly, in particle rigidity. An agreement was then made between NASA and NBS for NASA to provide a sufficient quantity of the 10 m material to make up 600 5 mL vials containing liquid suspensions of the polystyrene microspheres for use as an SRM. The agreement also called for NBS to receive the 30 m polystyrene spheres to be grown on a subsequent shuttle flight. To certify SRM 1960, the so-called “space beads” (Figs. 1 and 2), researchers at NBS, led by Tom Lettieri of the PED, developed three new particle-sizing techniques, which are described in the publication Certification of SRM 1960: Nominal 10 m Diameter Polystyrene Spheres (“Space Beads”) [1]. These techniques were center distance finding (CDF), resonance light scattering (RLS) from a liquid suspension of microspheres, and metrology electron microscopy (MEM). The primary certification technique for SRM 1960, center distance finding, was developed by Ike Hartman of the PED. CDF uses a conventional optical microscope and relies on the fact that the microspheres act like tiny lenses when placed on a glass slide in the microscope. The microspheres were spread onto a slide such that they formed long chains of contacting spheres, rather than the regular hexagonal arrays formed in conventional array sizing. This ensured that the particles were in close contact, compared to conventional array sizing where the non-zero diameter distribution of the microspheres leads to voids, cracks, gaps, and other flaws in the arrays. These flaws lead to uncertainties in