Setting the standard for subatomic forces

Everyday language makes plenty of references to “force” – a person with a strong personality is a ‘force of nature,’ or we worry about unseen ‘forces of evil.’ But the physics version came before all those figures of speech – force as a push or pull that changes the motion of an object. Accurate evaluation of physical force matters constantly in every day life – a car moves forward when its bulk is propelled by gas being burned in a combustion engine, or an arm thrusts a ball forward to the first baseman or wide receiver. The car only works if all its component parts, made in many different places, fit together properly, and if its chosen engine generates enough – you guessed it – force to move the assembled body.

So, how do people compare forces? Way back in high school science, you might have heard of Newton’s Second Law. The simple mathematical description of physical force boils down to a famous formula, in which Force = Mass multiplied by Acceleration. By convention, the unit of measurement of force is now the appropriately-named "Newton," which is defined as the force required to accelerate a 1 kilogram mass at a particular rate (1 square meter, to be exact). This standard definition, coupled with known reference materials like that kilogram, allows engineers in the United States to describe amounts of force in ways in the same as engineers in, say, Korea.

Interestingly, the kilogram remains the only international measurement unit that is defined by an arbitrary “artifactual” standard – a hunk of metal made in 1889 that resides outside of Paris. All the others, such as length, time, and temperature, are linked to some natural phenomenon that’s the same the world over. For example, the chief unit of time, the second, is linked to properties of cesium atoms, an “intrinsic” standard. The intrinsic approach has obvious advantages, unless you really find it convenient to travel to France every time you need to check the whether your kilogram of car parts is the same as someone else’s. That’s why researchers at the National Institute of Standards and Technology (NIST), a subdivision of the U.S. federal government’s Department of Commerce in Gaithersburg, MD, are working hard on a new definition for the kilogram (http://www.nist.gov/mel/mmd/mf/rekilo.cfm). And the problems are even harder when the objects involved are on the nanoscale, thousands of times smaller than an engine or a baseball.

Enter NIST researchers Jon Pratt and Gordon Shaw. Dr. Pratt and Dr. Gordon are developing a new and novel means of standardizing the definition of very small forces between atoms (http://www.nist.gov/mel/mmd/mf/sfmet.cfm). Their work, soon to be published in detail, was initially described during Dr. Pratt’s keynote address to the 2010 Annual Meeting of the Society for Experimental Mechanics (see http://www.nist.gov/manuscript-publication-search.cfm?pub_id=905416).

To create a new standard, they’ve studied the behavior of piece of double stranded DNA dissolved in a fluid under defined conditions of temperature and pH. Ordinarily DNA has a loose structure, rather like a woven rope that coils and flops randomly in solution. But the DNA can be anchored to a surface at one end, then picked up and pulled from the other end by a tiny lever in a large instrument designed to measure forces called an atomic force microscope (AFM). This arrangement simultaneously stretches out the DNA rope and measures the force being exerted, when the AFM lever senses resistance. When the pulling action first begins to stretch the DNA, the molecule resists the pull, and increasing force is measured via the AFM’s lever. But then at a certain point, the rope begins to unravel or fray instead of the resistance increasing – and for a fairly long period of more pulling, the force being measured doesn’t change. Eventually, when pulled hard enough and long enough, the rope puts up a last gasp of resistance and very large forces are measured, until the rope breaks apart altogether.

That intermediate zone, in which the force remains constant despite increasing pull, can be taken advantage of to use as a standard means of defining a particular amount of molecular force. “Our goal,” says Dr. Pratt, “is a recipe for people to be able to repeat anywhere in the world and get a known force.” The standard DNA can be made and used by any group anywhere in the world who has access to what are now relatively cheap and common materials and equipment. In fact, Dr. Shaw points out, “In an afternoon’s work you can make enough [DNA] to give everybody on the planet 5000 force references.” And the intrinsic property of that piece of DNA is the same whether stretched by an atomic force microscope in Montreal or in Moscow.

The technical catch is that the properties of the levers used in the atomic force microscope have to also be comparable and known, or “calibrated,” in order to get the same results everywhere. To solve that problem, Dr. Pratt built the world’s first electrostatic force balance (“Five years of my life,” jokes Dr. Pratt), a large and delicate piece of equipment that’s used to calibrate the levers used in atomic force microscopes. A calibrated set of levers can be shipped throughout the world to other reference agencies. Thus, NIST is on the forefront of furthering even more new applications for materials made on the nanoscale, which are now ubiquitous in products as varied as medicines, suntan lotion, and semi-conductors.