  We demonstrate levitation and three-dimensionally stable trapping of a wide
variety of particles in a vacuum chamber through the use of the thermophoretic
force in the presence of a strong temperature gradient. Typical sizes of the
trapped particles are between 10 microns and 1 mm at a pressure between 1 and
10 Torr. The trapping stability is provided by the geometry of the temperature
field, as well as the transition between the free molecule and hydrodynamic
regimes of the thermophoretic force. To quantitatively measure the
thermophoretic force, we examine the levitation heights of spherical
polyethylene spheres under various experimental conditions and determine the
temperature gradient needed to levitate the particles. A good agreement between
our experimental observations and theoretical calculations is obtained. Our
system offers a new platform to study thermophoretic phenomena and to simulate
dynamics of interacting many-body systems in a microgravity environment.
