Is Glass A Solid Or An Extremely Slow Moving Liquid?
    By News Staff | August 9th 2007 01:27 PM | 4 comments | Print | E-mail | Track Comments
    When most people look at a window they see solid panes of glass but to physicists it isn't so simple. Window glass has always been a puzzle but an Emory University research team led by physicist Eric Weeks has found another clue.

    Weeks has devoted his career to probing the mysteries of "squishy" substances that cannot be pinned down as a solid or liquid. Referred to as "soft condensed materials," they include everyday substances such as toothpaste, peanut butter, shaving cream, plastic and glass.

    Scientists fully understand the process of water turning to ice.

    As the temperature cools, the movement of the water molecules slows. At 32 F, the molecules form crystal lattices, solidifying into ice. In contrast, the molecules of glasses do not crystallize. The movement of the glass molecules slows as temperature cools, but they never lock into crystal patterns. Instead, they jumble up and gradually become glassier, or more viscous. No one understands exactly why.

    "One idea for why glass gets so viscous is that there might be some hidden structure," says Weeks, associate professor of physics. "If so, one question is what size is that structure".

    The Emory Physics lab began zeroing in on this question two years ago when Hetal Patel, an undergraduate who was majoring in chemistry and history, designed a wedge-shaped chamber, using glue and glass microscope slides that allowed observation of single samples of glassy materials confined at decreasing diameters.

    For samples, the Emory lab used mixtures of water and tiny plastic balls - each about the size of the nucleus of a cell. This model system acts like a glass when the particle concentration is increased.

    The samples were packed into the wedge-shaped chambers, then placed in a confocal microscope, which digitally scanned cross-sections of the samples, creating up to 480 images per second.

    The result was three-dimensional digital movies, showing the movement and behavior of the particles over time, within different regions of the chamber.

    "The ability to take microscopy movies has greatly improved during the past five to 10 years," Weeks says. "Back in the mid-90s, the raw data from one two-hour data set would be four gigabytes. It would have completely filled up your hard drive. Now, it's just a tiny part of your hard drive, like a single DVD."

    Two students collected and analyzed the data: Carolyn "Carrie" Nugent, an undergraduate from Bucknell University who worked in the Emory Physics Lab during two summers, and Kazem Edmond, currently an Emory graduate student in the Department of Physics.

    The data showed that the narrower the sample chamber, the slower the particles moved and the closer they came to being glass-like. When the researchers increased the particle concentration in the samples, the confinement-induced slowing occurred at larger plate separations. The dimension between the plates at which the particles consistently slowed their movement was 20 particles across.

    "It's like cars and traffic jams," Weeks says. "If you're on the highway and a few more cars get on, you don't really care because you can still move at the same speed. At 3 p.m., traffic gets worse and you may slow down a little bit. But at some point, your speed has to go from 40 mph to 5 mph. That's kind of what's happening with glass."

    Previous research has shown groups of particles in dense suspensions move cooperatively. "Our work suggests glasses are solid-like because these groups can't move when the sample chamber is thinner than the typical size of these groups," Weeks says. "These experiments help us understand earlier work done with thin polymer films and other glassy materials, but as we use particles rather than atoms, we get to directly see how confinement influences the glass transition."

    Nanotechnology is one example of a field that can benefit from research into the behavior of colloidal glass and plastics in tight spaces.

    "When making machines as small as a cell, people have found that they're even more fragile than you might expect," Weeks said. "One interesting thing is that small plastic structures become more fragile because, when they are really tiny, they're less glassy."



    Its been proven a solid. The theory of old glass being wavy therefore its a liquid, has 100% to do with the way they used to make glass, not with any "flowage" of glass molecules.

    Its been proven a solid.
    I'd like to see some documentation on this.

    I sent this SciBlog article to Adrian C. Wright, a long-time researcher in this field.  He sent me a recent collection  of articles, sponsored by Corning, from six "eminent glass scientists".  In his own article I read this:

    As the cooling progresses, eventually the atoms cannot even keep up with the optimum (supercooled) liquid structure and the material effectively solidifies into a glass. This process is known as the glass transition, and it is accompanied by a reduction in the rate at which the volume changes with decreasing temperature.  The temperature at which glass formation occurs is thus governed by the cooling rate, and whether a material is classified as a liquid or a solid depends on the time scale. This is illustrated by materials such as pitch and Silly Putty. When it is struck, pitch exhibits conchoidal fracture, while a ball of Silly Putty will bounce as an elastic solid if it is dropped onto the floor. On a longer time scale, both materials act as liquids and will flow, Silly Putty over a period of minutes and pitch during an even longer time. In this connection, it should also be noted that, contrary to popular belief, the glass in old churches and other windows has not flowed, since the extrapolated characteristic relaxation time at ambient temperatures is longer than the age of the universe! The reason why the glass is thicker at the bottom is that the original pane was not uniform and the window is more stable if the panes are assembled in this configuration.

    The last sentence is, I think, documentation enough!

    The collection also includes quotes in the margin from eminent glass scientists through the ages.  Here are two from since WW2:

    “Actually, the ways of fundamental investigations are inscrutable and the mysteries of glass are infinite!” — Evgenii Alexandrovich Porai-Koshits (1997)

    “Before our eyes, glass is changing from an amorphous, inert and dead mass into a substance full of riddles, unexpected anomalies, and as yet unutilized rich potential.” — Alexander Alexeevich Lebedev (1958) 

    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Something people often don't take account of is glass used be floated on lead sheets, and it affected the glass structure, so 'old glass' did sag over time, partly due to a lower melt temperature and continued sunlight
    It's little use testing modern glass then saying it's not liquid, as  for example, Pilkingtons make some amazingly complex glasses for modern buildings specially designed for torsional movement and windage in large panels
    A long way from 'old glass' for sure

    But the bottom line is, it depends what you want your glass for, as to whether its solid or liquid, and over what time period you measure