Photophoresis denotes the phenomenon that small particles suspended in gas

Gas

Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

 (aerosols) or liquids (hydrocolloids) start to migrate when illuminated by a sufficiently intense beam of light

Light

Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

. The existence of this phenomenon is owed to a non-uniform distribution of temperature of an illuminated particle in a fluid

Fluid

In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

 medium. Separately from photophoresis, in a fluid mixture of different kinds of particles, the migration of some kinds of particles may be due to differences in their absorptions of thermal radiation and other thermal effects collectively known as thermophoresis

Thermophoresis

Thermophoresis, thermodiffusion, or Soret effect , is a phenomenon observed when a mixture of two or more types of motile particles are subjected to the force of a temperature gradient and the different types of particles respond to it differently. The term "Soret effect" normally means...

. In laser photophoresis, particles migrate once they have a refractive index different from their surrounding medium. The migration of particles is usually possible when the laser is slightly or not focused. A particle with a higher refractive index compared to its surrounding molecule moves away from the light source due to momentum transfer from absorbed and scattered light photons. This is referred to as a radiation pressure

Radiation pressure

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...

 force. This force depends on light intensity and particle size but has nothing to do with the surrounding medium. Just like in Crookes radiometer

Crookes radiometer

The Crookes radiometer, also known as the light mill, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement...

, light can heat up one side and gas molecules bounce from that surface with greater velocity, hence push the particle to the other side. Under certain conditions, with particles of diameter comparable to the wavelength of light, the phenomenon of a negative indirect photophoresis occurs, due to the unequal heat generation on the laser irradiation between the back and front sides of particles, this produces a temperature gradient in the medium around the particle such that molecules at the far side of the particle from the light source may get to heat up more, causing the particle to move towards the light source.

Discovery of photophoresis is usually attributed to Felix Ehrenhaft

Felix Ehrenhaft

Felix Ehrenhaft was an Austrian physicist who contributed to atomic physics, to the measurement of electrical charges and to the optical properties of metal colloids. He was known for his maverick and controversial style...

 in the 1920s, though earlier observations were made by others including Augustin-Jean Fresnel

Augustin-Jean Fresnel

Augustin-Jean Fresnel , was a French engineer who contributed significantly to the establishment of the theory of wave optics. Fresnel studied the behaviour of light both theoretically and experimentally....

.

Applications of photophoresis

The applications of photophoresis expand into the various divisions of science, thus physics, chemistry as well as in biology. Photophoresis is applied in particle trapping and levitation, in the field flow fractionation of particles, in the determination of thermal conductivity and temperature of microscopic grains and also in the transport of soot particles in the atmosphere. The use of light in the separation of particles aerosols based on their optical properties, makes possible the separation of organic and inorganic particles of the same aerodynamic size.

The theory of photophoresis

Direct Photophoresis is caused by the transfer of photon momentum to a particle by refraction and reflection. Movement of particle in the forward direction occurs when the particle is transparent and has an index of refraction larger compared to its surrounding medium. Indirect Photophoresis occurs as a result of an increase in the kinetic energy of molecules when particles absorbs incident light only on the irradiated side, thus creating a temperature gradient within the particle. In this situation the surrounding gas layer reaches temperature equilibrium with the surface of the particle. Molecules with higher kinetic energy in the region of higher gas temperature impinge on the particle with greater momenta than molecules in the cold region; this causes a migration of particles in a direction opposite to the surface temperature gradient. The component of the photophoretic force responsible for this phenomenon is called the radiometric force. This comes as a result of uneven distribution of radiant energy (source function within a particle).
Indirect Photophoretic force, Fp depends on the physical properties of the particle and the surrounding medium.

External links

• Photophoresis in the context of astrophysics
  Astrophysics
  Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
  
  
• Negative photophoresis