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Crossed channels
The last word– CROSSED CHANNELS While cycling in Ireland I had ample opportunity to observe rain and puddles. I took a photo (below) of muddy water running across a road. Why has it separated into bands, and what determines their spacing?
● What is being observed is the effect that changing the velocity of a fluid has on any solids being transported by it. If the fluid is moving with a velocity above that at which the solid would normally settle out, localised currents in the fluid will stop the solid settling normally. I suspect the road has some surface imperfection which caused an initial drop in velocity, allowing some of the sediment load to settle onto the road surface as a band. The second band is then caused by the drop in velocity generated by the first band and so on, with the spacing determined by the gradient and roughness of the surface. The bands are curved because the water
Questions and answers should be kept as concise as possible. We reserve the right to edit items for clarity and style. Please include a daytime telephone number and a fax if you have one. Questions should be restricted to scientific enquiries about everyday phenomena. The writers of all answers that are published will receive a cheque for £25 (or the US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted
eventually builds enough pressure to force its way around the sides of each band of sediment. With this pressure comes velocity and so, once the velocity is high enough, no more sediment settles out in that band. The same principle of solids settling because of a drop in velocity is used to separate minerals such as gold from river sediments using a pan. Neil Ayre Kalgoorlie, Western Australia ● This is a result of a complex series of interactions involving, among other factors, the speed of the fluid flow, the viscosity of the fluid, and the ratio of the thickness of the top layer to the overall thickness of the fluid. This and related problems were first studied by Scott Russell in the early 19th century. In the later part of that century Diederik Korteweg and Gustav de Vries modelled the phenomenon with an equation that bears their names. The types of selfreinforcing waves formed in this situation are known as solitons and
by readers in any medium or format. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK (fax +44 (0) 20 7611 1280) or by email to [email protected] (all correspondents should include their postal address in order to receive payment for answers). If you would like a complete list of all unanswered questions please send an SAE to LWQlist at the above address.
Memorable answer? As part of our 50th anniversary we have teamed up with Crucial Technology (www.crucial.com/uk) and will be awarding each successful author a 512MB Gizmo! overdrive.
Last Words past and present, plus a full list of unanswered questions, are available on New Scientist’s website at www.newscientist.com
they are fundamentally different from normal waves occurring in bodies of fluid such as the sea, where the depth of the fluid is very large compared with the height of the wave crest. Doug Dean Pfeffingen, Switzerland
effect is lost. Heaping is then reliant solely on particle packing, resulting in a smaller mound. Incidentally, older coffee that has lost a significant proportion of its oil doesn’t heap well. Ross Rounsevell Cambridge, UK
● This occurs when water or other fluids run in a thin film over a surface. Male readers may sometimes see the effect when using urinals. Tony Cowley By email, no address supplied
● Roasting changes coffee chemically, producing thousands of compounds in solid, liquid and gaseous forms. Charred structural components, such as cellulose and lignin, become brittle and rigid. Proteins and gummy carbohydrates form tarry composites – viscoelastic when warm; glassy and brittle when cold. Roasting also parches the coffee, so the gummy materials shrink and distort. Grinding the coffee fluffs it out, loosely piling up flaky, irregular particles, and sealing ground coffee into bags or containers causes compaction. Freezing the compacted coffee then fixes the viscoelastic materials into their stressed shapes. As that liberated teaspoon of ground coffee warms up and absorbs moisture from the air in the absence of compression, the viscoelastic components soften and begin to yield under the stress of the elastic components. The grains gradually recover their shapes. Jon Richfield Somerset West, South Africa
GROWING SPOONFUL I keep my ground coffee in the freezer. If I take a teaspoonful straight from the freezer, its shape is low and rounded. Once the coffee has reached room temperature – about 25 °C here in the tropics – a teaspoon of it forms an elevated peak of larger volume. Why?
● The roasting process for coffee is straightforward, but the development of flavour during the roasting is more complex. Roasting results in a significantly desiccated bean suffused with volatile “coffee oil” (not really an oil because it is readily soluble in water) that is concentrated during the roasting process. At ambient temperatures the coffee grounds would associate by a mixture of particle packing and, significantly, adhesion between particles caused by the coffee oil. This allows the coffee to heap surprisingly high on the spoon. At the lower temperatures typical of a freezer, the coffee oil freezes and the adhesion
THIS WEEK’S QUESTION Keeping pace If I was poisoned with a drug to stop my heart beating, would my pacemaker keep me alive? Mark Rowney By email, no address supplied
Does Anything Eat Wasps? New Scientist’s latest collection from The Last Word, answering some of the world’s most baffling questions Available in bookstores and online
● What is being observed is the effect that changing the velocity of a fluid has on any solids being transported by it. If the fluid is moving with a velocity above that at which the solid would normally settle out, localised currents in the fluid will stop the solid settling normally. I suspect the road has some surface imperfection which caused an initial drop in velocity, allowing some of the sediment load to settle onto the road surface as a band. The second band is then caused by the drop in velocity generated by the first band and so on, with the spacing determined by the gradient and roughness of the surface. The bands are curved because the water
Questions and answers should be kept as concise as possible. We reserve the right to edit items for clarity and style. Please include a daytime telephone number and a fax if you have one. Questions should be restricted to scientific enquiries about everyday phenomena. The writers of all answers that are published will receive a cheque for £25 (or the US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted
eventually builds enough pressure to force its way around the sides of each band of sediment. With this pressure comes velocity and so, once the velocity is high enough, no more sediment settles out in that band. The same principle of solids settling because of a drop in velocity is used to separate minerals such as gold from river sediments using a pan. Neil Ayre Kalgoorlie, Western Australia ● This is a result of a complex series of interactions involving, among other factors, the speed of the fluid flow, the viscosity of the fluid, and the ratio of the thickness of the top layer to the overall thickness of the fluid. This and related problems were first studied by Scott Russell in the early 19th century. In the later part of that century Diederik Korteweg and Gustav de Vries modelled the phenomenon with an equation that bears their names. The types of selfreinforcing waves formed in this situation are known as solitons and
by readers in any medium or format. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK (fax +44 (0) 20 7611 1280) or by email to [email protected] (all correspondents should include their postal address in order to receive payment for answers). If you would like a complete list of all unanswered questions please send an SAE to LWQlist at the above address.
Memorable answer? As part of our 50th anniversary we have teamed up with Crucial Technology (www.crucial.com/uk) and will be awarding each successful author a 512MB Gizmo! overdrive.
Last Words past and present, plus a full list of unanswered questions, are available on New Scientist’s website at www.newscientist.com
they are fundamentally different from normal waves occurring in bodies of fluid such as the sea, where the depth of the fluid is very large compared with the height of the wave crest. Doug Dean Pfeffingen, Switzerland
effect is lost. Heaping is then reliant solely on particle packing, resulting in a smaller mound. Incidentally, older coffee that has lost a significant proportion of its oil doesn’t heap well. Ross Rounsevell Cambridge, UK
● This occurs when water or other fluids run in a thin film over a surface. Male readers may sometimes see the effect when using urinals. Tony Cowley By email, no address supplied
● Roasting changes coffee chemically, producing thousands of compounds in solid, liquid and gaseous forms. Charred structural components, such as cellulose and lignin, become brittle and rigid. Proteins and gummy carbohydrates form tarry composites – viscoelastic when warm; glassy and brittle when cold. Roasting also parches the coffee, so the gummy materials shrink and distort. Grinding the coffee fluffs it out, loosely piling up flaky, irregular particles, and sealing ground coffee into bags or containers causes compaction. Freezing the compacted coffee then fixes the viscoelastic materials into their stressed shapes. As that liberated teaspoon of ground coffee warms up and absorbs moisture from the air in the absence of compression, the viscoelastic components soften and begin to yield under the stress of the elastic components. The grains gradually recover their shapes. Jon Richfield Somerset West, South Africa
GROWING SPOONFUL I keep my ground coffee in the freezer. If I take a teaspoonful straight from the freezer, its shape is low and rounded. Once the coffee has reached room temperature – about 25 °C here in the tropics – a teaspoon of it forms an elevated peak of larger volume. Why?
● The roasting process for coffee is straightforward, but the development of flavour during the roasting is more complex. Roasting results in a significantly desiccated bean suffused with volatile “coffee oil” (not really an oil because it is readily soluble in water) that is concentrated during the roasting process. At ambient temperatures the coffee grounds would associate by a mixture of particle packing and, significantly, adhesion between particles caused by the coffee oil. This allows the coffee to heap surprisingly high on the spoon. At the lower temperatures typical of a freezer, the coffee oil freezes and the adhesion
THIS WEEK’S QUESTION Keeping pace If I was poisoned with a drug to stop my heart beating, would my pacemaker keep me alive? Mark Rowney By email, no address supplied
Does Anything Eat Wasps? New Scientist’s latest collection from The Last Word, answering some of the world’s most baffling questions Available in bookstores and online