### Rough Microparticles

July 17, 2017

Billiard balls are smooth, since they are designed to roll on a flat surface, but most other balls are rough. In sports play, such as basketball and American football, the roughness helps the player to grip the ball. The dimpled surface of a golf ball, however, serves a different purpose. A golf ball's dimples aid the lift of a spinning ball, a fact discovered accidentally in 1930.[1-2] They create a thin turbulent boundary layer of air at the ball's surface that allows air to flow farther to the backside of the ball and decrease wake.[1-2]

This lift, called Magnus lift, is experienced by spinning bodies in a fluid medium, so it will occur for a ball thrown underwater. Unfortunately, if the golf ball is not properly hit, and there's sideways spin, it will also hook or slice. Magnus lift wasn't a problem for astronaut, Alan Shepard, who hit golf balls on the Moon during the Apollo 14 mission in 1971 using a six iron head attached to the handle of a scoop used for a rock sampler. The two actual golf balls that he used are still there.[3]

Figure one of US Patent No. 701,736, "Golf-Ball," by Eleazer Kempshall, June 3, 1902.[4]

In this case, the ball appears to have bumps, not dimples. The beneficial effect of dimples wasn't discovered until 1930.[2]

While the The Royal and Ancient Golf Club of St Andrews and the United States Golf Association has specifications that purposely limit golf ball performance, the only limitation relating to dimples is that a ball's speed must be less than 270 km/h (250 feet per second) for a particular test condition. There's no limitation on the number of dimples, with numbers around 350 being typical, with a variety of shapes and depths.

Whenever lift is involved, we need to invoke the name of Swiss mathematician, Daniel Bernoulli (1700-1782) and his eponymous Bernoulli's principle. I must confess that I often confuse Daniel with Jacob Bernoulli (1655-1705), since there are so many members of the Bernoulli family. Lift is given as a function of the differential pressure p on a body, as follows,

In which v is the velocity of the fluid (air) flowing around the body, g is the gravitational acceleration, z is the height of the body, ρ is the density of the body, and the the constant depends on the important parameters of the specific case. While a golf ball will exit the tee at about 80 meters/sec with a spin of about 60 rev/sec, getting the equation parameters takes a bit of experimentation, or a lot of computer modeling. Let's just say, as many of my professors were wont to do, that the rest is left as an exercise for the interested reader.

ignoring for the moment, since hunting is a sport, the possible inclusion of birdshot (2-4 mm) as a sports ball, we have a panoply of ball examples from squash balls (40 mm diameter) through basketballs (755 mm). As Richard Feynman so presciently said in 1959, "there's plenty of room at the bottom." An international team of scientists from ETH Zürich (Zürich, Switzerland), the Leibniz Institute of Polymer Research (Dresden, Germany), and Sofia University (Sofia, Bulgaria) has been creating silica (SiO2) microparticles with tunable surface texture, some of which resemble raspberries and spherical colonies of bacteria.[5-6]

silica raspberries.

scanning electron microscope image of the silica microparticles, showing their controlled surface roughness.

(ETH Zurich image by Michele Zanini, Isa Group.)

These silica "raspberries" were created as an additive to stabilize emulsions, which are mixtures of two immiscible liquids, such as oil in water. In an oil-in-water emulsion, small droplets of oil are dispersed in water.[6] Non-stabilized emulsions will separate, as the dispersed droplets merge into larger droplets. A number of chemical emulsifiers are available to prevent this, such as surfactants, polymers, and proteins.[6] A different type of emulsifier is needed for the two different cases of a oil-in-water emulsions and water-in-oil emulsions. The silica raspberries work in both cases.[6]

In the 1900's, the British chemists, W. Ramsden and S. U. Pickering, found that emulsions could be stabilized by very fine solid particles, such as spherical silica particles.[6] In the present study, the research team prepared silica spheres of 1-6 micrometer diameter with a roughened surface of smaller diameter silica nanoparticles. They were able to tune the surface roughness to create a range of candidate emulsifier particles.[6] The idea is that surface heterogeneities, such as roughness, will significantly affect the adsorption, motion and interactions of particles at the surface.[5]

The rough particles were created by electrostatically adsorbing negatively charged silica nanoparticles of 12–250 nm diameter onto the larger, 1-6 μm silica core microparticles that were positively charged after a surface modification.[5] A modified Stöber process was used to fill the gaps between smaller surface spheres and seal them together.[5] a library of particles with a broad range of surface roughness was obtained by selection of the size ratio of the core and surface particle diameters, and the thickness of the grown silica layer.[5]

A diagram of the fabrication process for the rough silica spheres. (Fig. 1(a) of ref. 5, modified for better display, and licensed under a Creative Commons Attribution 4.0 International License.)[5)]

The surface roughness was characterized using atomic force microscopy, and the root-mean-squared roughness ranged from 1, for an unmodified core, to 54 nm for particles on a 6 μm core.[5] It was found that these particles will stabilize both water-in-oil and oil-in-water emulsions.[6] An intermediate roughness provided maximum emulsion stability and adsorption efficiency.[5] A patent application has been filed for production of such particles as emulsion stabilizers, and there are many applications in foods, cosmetics, and pharmaceuticals.[6]

### References:

Linked Keywords: Billiard ball; smooth; rough; sports play; basketball; American football; golf ball; lift; angular momentum; spin; serendipity; accidentally; turbulence; turbulent; boundary layer; atmosphere of Earth; air; wake; Magnus effect; Magnus lift; fluid medium; underwater; golf stroke-hook; golf stroke-slice; astronaut; Alan Shepard; Moon; Apollo 14 mission; six iron; handle; shovel; scoop; rock; Google Patents; The Royal and Ancient Golf Club of St Andrews; United States Golf Association; speed; kilometers per hour; km/h; feet per second; Switzerland; Swiss; mathematician; Daniel Bernoulli (1700-1782); eponymous; Bernoulli's principle; Jacob Bernoulli (1655-1705); Bernoulli family; function; differential pressure; velocity; airflow; gravitational acceleration; altitude; height; density; parameter; tee; meters/sec; rotation; rev/sec; equation; experiment; experimentation; computer simulation; computer modeling; professor; wont to do; left as an exercise for the interested reader; hunting; sport; birdshot; panoply; squash ball; diameter; Richard Feynman; prescient; presciently; there's plenty of room at the bottom; scientist; ETH Zürich (Zürich, Switzerland); Leibniz Institute of Polymer Research (Dresden, Germany); Sofia University (Sofia, Bulgaria); silicon dioxide; silica (SiO2); microparticle; surface finish; surface texture; raspberry; raspberries; colony (biology); bacteria; scanning electron microscope; surface roughness; Michele Zanini, Isa Group; emulsion; mixture; miscibility; immiscible; oil; water; droplet; separation process; dispersion (chemistry); dispersed; chemical compound; emulsifier; surfactant; polymer; protein; Great Britain; British; chemist; Pickering emulsion; W. Ramsden and S. U. Pickering; solid; research; micrometer; nanoparticle; heterogeneity; adsorption; motion; Coulomb's law; interaction; electrostatics; electrostatically; negative; electric charge; positive; Stöber process; ratio; schematic; diagram; Creative Commons Attribution 4.0 International License; atomic force microscopy; root-mean-square; patent application; food; cosmetic; pharmaceutical.