A particle in motion in a fluid causes eddies which slowly spread out, and in doing so also causes a sound wave (a density wave). That sound is then rapidly transmitted further and after a short time only an eddy remains in the fluid in the vicinity of the particle. However, computer simulations at the Institute of Atomic and Molecular Physics at the NWO's Foundation for Fundamental Research on Matter (FOM) in Amsterdam have shown that precisely the opposite occurs when the particle is contained in a narrow tube. The eddy is then quickly absorbed but the sound wave remains and strangely enough pushes the particle back in the opposite direction.
The Dutch physicists observed this phenomenon during computer simulations of the behaviour of suspensions of small colloidal particles. Such particles have diameters from a thousandth of a millimetre down to a hundred thousandth and are found in such products as milk, paint, blood or mayonnaise. In the course of their calculations, the researchers observed a narrow tube containing a fluid and a colloidal particle. The initial direction of speed of the particle was along the axis of the tube.
The surprising reversal of the particle's direction of speed occurred only in narrow tubes whose diameter was only three to five times that of the particle. Normally, low-frequency sound in fluids is transmitted over great distances; whales use such sound for communication, for example.
In a narrow tube, however, the situation is different. When the colloidal particle begins to move, the fluid in front of it is compressed to some extent, similarly to the bow wave of a ship. Under normal circumstances, the density wave is transmitted at the speed of sound. However, the walls of the tube prevent this from happening. The sound (particularly the low frequencies) therefore remains virtually stationary. The density 'in front of' the particle, which is greater than 'at the back' of it, induces a flow in the vicinity of the colloid which at a certain point reverses the direction of motion of the particle.
The phenomenon of low-frequency sound not being transmitted well in narrow tubes is already a familiar one. When a sailor calls "Aye, Aye Sir" down the ship's voice tube, his voice is distorted because of the way in which the low frequencies within it are muffled.
Further information:
dr. Maarten Hagen (AMOLF)
T +31 20 608 1234, F +31 20 668 4106
e-mail hagen@amolf.nl