Particle sizing technique

• US 6,219,138 B1
• Filed: 01/10/2000
• Issued: 04/17/2001
• Est. Priority Date: 01/10/2000
• Status: Expired due to Fees

• Current Assignee: the united states of america as represented by the secretary of the navy
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Billard, Barton D., Swanson, Nancy L.
• Primary Examiner(s): Pham, Hoa Q.
• Assistant Examiner(s): Nguyen, Sang H.
• Application Number:
  US09/480,535
• Time in Patent Office: 463 Days
• Field of Search: 356/335, 356/336, 356/337, 356/338, 356/339, 356/340, 356/341, 356/342, 356/343, 250/574, 250/575
• US Class Current: 356/336
• CPC Class Codes: G01N 15/0205 by optical means

Abstract

The particle size within a given medium is determined using a single wavelength to confirm the known particles sizes within a dispersion, or determined from two wavelengths to calculate the unknown particle sizes of a dispersion. Three wavelengths may be used to determine the unknown particle size of a dispersion of unknown concentration within a medium. The method and apparatus may be used for mono-dispersions and poly-dispersions.

Claims

1. A method for determining particle size by measuring optical transmission, comprising the steps of:

calculating a ratio of the volume of solids to a total volume of particles and medium (R) of a sample solution of a solid dispersion within a medium;

determining an optical thickness (τ

) of the sample solution at one or more wavelengths;

generating the extinction (Q) as a function of size parameter x for the one or more wavelengths; and

,

comparing a measured value of either optical thickness or volume ratio at a first wavelength to a predicted value, with the predicted value dependent on either the measured volume fraction ratio of the sample solution, or on measurement of the optical thickness between the value of about 0 to about 10.

2. The particle sizing technique of claim 1, wherein the step of calculating the ratio of solids within a first sample solution comprises measuring an amount of dilution of the solids to form the first sample solution.

3. The particle sizing technique of claim 1, wherein the step of determining the optical thickness of the sample solution comprises measuring the intensities of an electromagnetic source with and without the sample solution in a beam path of the electromagnetic source.

4. The particle sizing technique of claim 3, wherein the step of determining the optical thickness of the sample solution comprises calculating the optical thickness from the relationship of:

5. The particle sizing technique of claim 1, wherein the step of generating the extinction Q as a function of size parameter x comprises numerically generating Q(x) from a Mie code using the ratio of the refractive index of the particle divided by a refractive index of the medium as input.

6. The particle sizing technique of claim 1, further comprising the steps of:

generating the predicted volume fraction ratio of the first sample solution;

calculating the measured value of R of the first sample solution; and

,

determining the common values of the predicted volume fraction ratio and the measured value of R, wherein at least one determined common value represents the particle size of the solids within the sample solution.

7. The particle sizing technique of claim 6, wherein the step of generating the predicted volume fraction ratio comprises a determined value generated from the Mie theory output Q(x).

8. The particle sizing technique of claim 6, wherein the step of generating the predicted volume fraction ratio comprises a determined value generated from the Mie theory output Q(x) and the relationship of R=2λ

τ

x/3π

zQ(x), wherein R is the predicted volume fraction occupied by the solids in the sample solution, λ

is the wavelength of the radiation in the medium, τ

is the optical thickness of the sample, x is size parameter, z is the optical path length, and Q is the scattering efficiency.

9. The particle sizing technique of claim 6, wherein the step of determining the common values of the predicted volume fraction ratio and the measured value of R comprises plotting R(x) as a function of x.

10. The particle sizing technique of claim 6, wherein the step of determining the common values of the predicted volume fraction ratio and the measured value of R comprises extending a constant line at the measured value of R.

11. The particle sizing technique of claim 6, wherein the step of determining the common values of the predicted volume fraction ratio and the measured value of R comprises plotting R(x) as a function of x and extending a constant line at the measured value of R to correlate the common values.

12. The particle sizing technique of claim 1, wherein the predicted optical thickness is calculated as a function of size parameter.

13. The particle sizing technique of claim 12, wherein the predicted optical thickness comprises a calculation from the equation:

14. The particle sizing technique of claim 12, wherein the step of determining the common values of the predicted optical thickness and the measured value of τ

comprises plotting τ

(x) as a function of x.

15. The particle sizing technique of claim 12, wherein the step of determining the common values of the predicted optical thickness and the measured value of τ

comprises extending a constant line at the measured value of τ

.

16. The particle sizing technique of claim 12, wherein the step of determining the common values of the predicted optical thickness and the measured value of τ

comprises plotting τ

(x) as a function of x and extending a constant line at the measured value of τ

to correlate two common values.

17. A confirmed particle size measurement of a sample of particles of known size resulting from the use of a single wavelength comprising the method of claim 1.

18. The particle sizing technique of claim 1, further comprising the steps of:

making an additional measurement at a second wavelength;

converting Q(x) to Q(d) for the first and second wavelength; and

,

comparing the measured value at the first wavelength to the predicted value at the first wavelength and the measured value at the second wavelength to the predicted value at the second wavelength.

19. The particle sizing technique of claim 18, wherein the step of making an additional measurement at a second wavelength comprises adjusting the sample solution to an optical thickness equal to the determined optical thickness at the first wavelength with the addition of solid or medium to the sample.

20. The particle sizing technique of claim 18, further comprising the step of generating the predicted volume fraction ratio as a function of Q(d) for the first and second wavelength.

21. The particle sizing technique of claim 20, wherein the step of generating the predicted

volume fraction ratio as a function of Q(d) comprises:

converting x to d and calculating the measured value of R for the first and second wavelength; and

,

the step of comparing the measured value at the first wavelength to the predicted value at the first wavelength and the measured value at the second wavelength to the predicted value at the second wavelength comprises determining the common values of the predicted volume fraction ratio and the measured value of R for each wavelength, wherein at least one determined common value for a given d represents the particle size of the solids within the sample solution.

22. The particle sizing technique of claim 21, wherein the step of generating the predicted volume fraction ratio as a function of Q(d) comprises the equation:

23. The particle sizing technique of claim 18, wherein the step of comparing the measured value at the first wavelength to the predicted value at the first wavelength and the measured value at the second wavelength to the predicted value at the second wavelength comprises plotting the predicted volume fraction ratio of the first and second wavelengths as functions of sphere diameter.

24. The particle sizing technique of claim 21, wherein the step of determining the common values of the predicted volume fraction ratio R(d) and the measured value of R comprises a graphic representation.

25. The particle sizing technique of claim 18, further comprising the step of generating the predicted optical thickness as a function of Q(d) for the first and second wavelength.

26. The particle sizing technique of claim 25, wherein the step of generating the predicted optical thickness as a function of Q(d) comprises the equation:

27. The particle sizing technique of claim 18, wherein the first sample solution is used as the second sample solution, so that the volume ratio R is the same for both measurements of τ

.

28. The particle sizing technique of claim 18, wherein the step of comparing the measured value at the first wavelength to the predicted value at the first wavelength and the measured value at the second wavelength to the predicted value at the second wavelength comprises plotting the predicted optical thickness of the first and second wavelengths as functions of the sphere diameter.

29. The particle sizing technique of claim 28, further comprising the step of determining the common values of the predicted optical thickness τ

(d) and the measured value of τ

.

30. The particle sizing technique of claim 1, wherein the sample solution comprises a monodispersion.

31. A particle size measurement of a sample of particles of unknown size determined from the use of two wavelengths comprising the method of claim 18.

32. The particle sizing technique of claim 1, further comprising generating a second Q(x) from a second refractive index as an input to the Mie code when the refractive index of a particle or medium is different at a second wavelength.

33. The particle sizing technique of claim 1, further comprising generating a second Q(x) from a second medium wherein the refractive index of the two media are different.

34. The particle sizing technique of claim 1, further comprising the steps of:

calculating the particle radius as a volume-surface mean radius.

35. The particle sizing technique of claim 34, wherein the sample solution comprises a polydispersion.

36. The particle sizing technique of claim 34, wherein the particle radius (r₃₂) is defined by the equation:

$r_{32}=\frac{{\int <br><br>}_0^{\mathrm{\infty <br><br>}}<br><br>N<br><br>\left(r\right)<br><br>r^3<br><br><br><br>r}{{\int <br><br>}_0^{\mathrm{\infty <br><br>}}<br><br>N<br><br>\left(r\right)<br><br>r^2<br><br><br><br>r}$

wherein N(r) is the size distribution function.

37. The particle sizing technique of claim 1, wherein the scattering efficiency Q comprises a mean value Q(r₃₂) defined by the equation:

$Q<br><br>\left(r_{32}\right)=\frac{{\int <br><br>}_0^{\mathrm{\infty <br><br>}}<br><br>Q<br><br>\left(r,m\right)<br><br>N<br><br>\left(r\right)<br><br>r^2<br><br><br><br>r}{{\int <br><br>}_0^{\mathrm{\infty <br><br>}}<br><br>N<br><br>\left(r\right)<br><br>r^2<br><br><br><br>r}$

wherein N(r) is the size distribution function.

38. The particle sizing technique of claim 1, comprising a ratio method using three wavelengths wherein the concentration of solids in the sample solution is unknown.

39. The particle sizing technique of claim 38, wherein the ratio method using three wavelengths comprises comparing two or more of the ratios of:

τ

(λ

₁)/τ

(λ

₂)=Q(λ

₁,d)/Q(λ

₂,d);

τ

(λ

₁)/τ

(λ

₃)=Q(λ

₁,d)/Q(λ

₃,d);

and τ

(λ

₂)/τ

(λ

₃)=Q(λ

₂,d)/Q(λ

₃,d),

wherein λ

₁ is the first wavelength, λ

₂ is the second wavelength, and λ

₃ is the third wavelength, and τ

(λ

₁) is the optical thickness at the first wavelength, τ

(λ

₂) is the optical thickness at the second wavelength, and τ

(λ

₃) is the optical thickness at the third wavelength.

40. The particle sizing technique of claim 39, wherein two or more ratios at three wavelengths are plotted as a function of d.

41. An unknown particle size of a sample of unknown concentration determined from the use of three wavelengths comprising the method of claim 38.

42. An apparatus having a solid angle of acceptance θ

_½

≦

5°

λ

/d for determining particle size by the method of claim 1.

43. A method for determining particle size by measuring optical transmission, comprising the steps of:

calculating a ratio of the volume of solids to the volume to the total volume of particles and medium (R) of a sample solution of a solid dispersion within a medium;

determining an optical thickness (τ

) of the sample solution at one or more wavelengths;

generating the extinction (Q) as a function of size parameter x for the one or more wavelengths; and

,

comparing a measured value of either optical thickness or volume ratio at a first wavelength to a predicted value, with the predicted value dependent on either the measured volume fraction ratio of the sample solution, or on measurement of the optical thickness between the value of about 0 to about 10;

wherein the step of calculating the ratio of solids within a first sample solution comprises measuring an amount of dilution of the solids to form the first sample solution;

wherein the step of calculating the measured value of R comprises the relationship of R=s/δ

, with a measured value of τ

, wherein R is the volume fraction occupied by the solids in the sample solution, s is the fraction of solids, and δ

is a density of the particles.