Patent Application: US-27406008-A

Abstract:
a method of measuring sizes of particles suspended in a liquid using optical mixing spectroscopy of scattered light and an apparatus for implementing the method are disclosed . the method involves making measurements by a fiber optic probe introduced into a medium under investigation , the probe including several multiple or single mode optical fibers . one of the optical fibers transmits light into the medium , while the other optical fibers transmits scattered light to a device for providing spatial coherence of the light and then to a light detector . auxiliary optical fibers are used to determine the relationship between spectral line widths of multiple and single light scattering when measurements are conducted in very turbid media and for determining particle sizes from multiple rather than single scattering spectrum .

Description:
fig2 shows a schematic diagram of an apparatus for implementing a method in accordance with the present invention , the apparatus comprising : a laser 1 , a laser - optical fiber interface 17 , an illuminating fiber 11 ; an fiber optic probe 14 ; a reservoir 5 with a suspension or emulsion under investigation ; a primary collecting optical fiber 12 ; an auxiliary collecting optical fiber 13 ; a collecting fiber - light detector interface 18 ; a displacing coupling 16 ; distance l between a fiber output end and an aperture diaphragm ; an aperture diaphragm 7 ; an objective or lens 3 ; a cathode diaphragm 15 ; a light detector 9 ; a correlator or spectrum analyzer 10 ; a coherence area selection system 19 . the present method is implemented by the apparatus in the following manner . an optical fiber output end is embedded in a thin probe 14 of cylindrical or another convenient shape along with an input end of a collecting multimode fiber 12 . the step of embedding is provided such that the fibers 11 and 12 should be parallel and in immediate vicinity from each other so that side surfaces thereof contact each other . working surfaces of the optical fibers are jointly polished . the resulting three - fiber probe 14 is placed into a volume 5 with a suspension under investigation ( colloidal solution , emulsion , etc .). light emitted from the fiber 11 illuminates the investigated medium volume in a cone shape with angular opening a ≈ 15 ° for a conventional optical fiber having the core diameter of about 100 μm . light back - scattered from particles ( more precisely , at scattering angle θ = 178 - 180 °) enters the collecting fiber 12 and is directed to a square - law light detector 9 that is mounted together with output ends of fibers 12 and 13 , lens 3 and diaphragms 7 , 15 in a collecting fiber - light detector interface 18 . a coherence area selection system 19 includes a lens 3 and a diaphragm 7 . then , spectral scattering line width γ is determined using optical mixing spectroscopy of scattered light , and radius r of particles is determined therefrom using ( 1 ). scattering volume is defined by the intersection between the first fiber illuminated volume and the second fiber field of view , and reaches 0 . 7 · 10 − 2 cm 2 ; this immediately overcomes the problem of non - gaussian statistics of dust scattering owing to the increased scattering volume , and solves the task of protection against the dust effect . light source is an endface of the fiber 12 ( typically , with d s ≈ 100 μm diameter ). to reduce exposure , an additional diaphragm is mounted on the light detector cathode , the diaphragm size should be not less than the fiber endface image on the light detector cathode . the aperture diaphragm diameter is defined using relationship ( 2 ) to obtain a single coherence area . in practice , an aperture diaphragm 7 having d a = 0 . 9 mm at l = 10 cm is sufficient to obtain 30 % coherence ratio ( correlation function amplitude / 0 . 30 background ratio ). particle sizes of a diluted latex solution , measured in a test tube by the invented method and by a conventional method agree , within the limits of error ( 1 . 5 %), with each other and with the certified value of latex sphere radius , 100 ± 2 nm . where the invented apparatus is used to make measurements in very turbid media , such as milk or emulsion , emulsol emu - 1 ( liquid lubricant - coolant ), the correlator detects the correlation function of multiple scattered light , rather than that of a single scattered one , for which relationship ( 1 ) is true . in this case the correlation function has near - exponential shape , but its width may differ from that of the correlation function of a single - scattered light several times . the difference depends on photon path length 1 in the medium or extinction ratio σ ( σ = 1 / 1 ). σ and 1 are explicitly related with scattering coefficient r and scattering ratio . to obtain information about values σ and 1 and the relation between single and multiple scattering line widths , an end of one or more auxiliary fibers 13 is inserted in the probe end some distance away ( typically 0 . 5 - 1 . 5 mm ) from and parallel to the illuminating and collecting fibers ( fig2 ). fibers ends are jointly polished in a single plane . output ends of optical fibers 12 , 13 , and so on , are secured in an optical fiber - light detector interface in a laterally moving coupling ( 16 in fig2 ) to enable light measurements in any one of the optical fibers . use of a diaphragm 15 in front of the light detector cathode is obligatory in this case since it is necessary to alternately measure light output from the collecting and auxiliary optical fibers . relationship between single and multiple scattering line widths is determined based on the relation of intensities and spectral line widths in collecting , i pr , γ pr , and auxiliary , i aux , γ aux , fibers , and the respective coefficient is introduced in the result obtained from formula ( 1 ). distance between centers of the optical fibers 11 and 12 is 0 . 27 mm , and between centers of the optical fibers 11 and 13 is 1 . 1 mm . fig3 shows a universal dependence of normalized particle radius r pr / r on r aux / r pr ratio . emulsions : ▪ emu - 1 , diluted to 10 %, r = 118 . 4 nm ; ▾ tafol , diluted to 5 %, r = 83 . 38 nm ; ▴ latex , r = 103 . 6 nm ; ● emu - 1 , concentration 5 . 5 %, r = 118 . 3 nm . big round point on the top right side of the plot corresponds to the case of infinite dilution and absence of multiple scattering , where r aux = r pr − r . this dependence is universal for particles of various radii and different materials , but it changes with variation in distances between the collecting , illuminating and auxiliary optical fibers , so it should be determined for each probe individually . values a and b in the dependence are approximated from the measured data by formula : therefore , with a fiber optic probe made as disclosed in the description and a universal dependence of r pr / r on r aux / r pr ratio determined for the probe , true particle size r can be obtained from r aux and r pr values measured in the collecting and auxiliary fibers both in transparent and turbid emulsions and suspensions using formula : a measurement method in accordance with the invention makes it possible to dispense with special optical cells . a fiber optic probe can be placed in a reservoir of any size , a pipeline , etc ., where the need for particle measurement exists . the optical system associated with a sample doesn &# 39 ; t require alignment , and the coherence area selection system can be adjusted and fixed before starting the apparatus service . measurements can be conducted both in transparent and very turbid media , such as milk or 5 - 10 % colloidal solution of emulsol ( liquid lubricant - coolant , llc ). in this case correlation function of multiple , rather than single light scattering is determined , whose coherence time γ m is related with coefficient γ defined on the basis of line width / intensity ratios in collecting and auxiliary fibers ( fibers 12 and 13 in fig2 ). owing to selection of a greater coherence area , a method in accordance with the invention is immune to dust present in a sample . 1 . photon correlation and light beating spectroscopy . ed . by h . z . cummins , e . r . pike , plenum press , new york — london , 1974 . translation . ed . by g . cumming and e . pike , mir , moscow , 1978 . 2 . kovalenko k . v ., krivohizha s . v ., rakaeva g . v ., chaikov l . l ., method and apparatus for preparing colloidal solutions , ru no . 2306970 of 21 dec . 2006 . 3 . malcolmson a . p ., holve d . j . in - line particle size measurements for cement and other abrasive process environments . proc . of ieee / pca 40 th cement industry technical conference , rapid city , s . dak ., 1998 ( attached ). 4 . o &# 39 ; 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