Experiment Results: Speckle Size Decreasing with Increase in Concentration

[Daymare]

I conducted an experiment in which I suspended 3 micron polybeads in an aqueous solution and shone laser light through them and calculated the speckle size(experiment done for concentration assessment).The result I obtained showed speckle size decreasing with increase in concentration.Can anyone give me a logical explanation as to why this is.

 

[Dale]

Does the speckle size also change if you move the optical system closer or further (different from the amount expected for distance)?

 

[Daymare]

I hadn't changed the distance between the detector and the chamber in which the aqueous solution is kept.Moreover the detector is kept at an angle to of 15 degrees to the chamber.But I think the speckle size would increase if I increase the distance between the detector and the chamber.Here since I have kept the detector at an angle,its scattering by the particles that causes the speckles rather than laser passing through an aperture.The thing that I need to know is does the formula for objective speckle size hold in this case.If so wouldn't the aperture size decrease with increase in concentration.Then the speckle size should increase right?But experimentally it seems to decrease.

 

[Dale]

I am certainly not an expert in speckles, so please take all my comments with appropriate skepticism.

My understanding is that speckles are primarily an interference phenomenon. So a higher concentration would mean a closer mean distance between scatterers. I. Would expect that leads to a larger speckle since the wavefronts will be closer.

However, I don't know how the detector angle will affect it and I don't know how using objective speckle instead of subjective speckle changes it.

 

[Andy Resnick]

I conducted an experiment in which I suspended 3 micron polybeads in an aqueous solution and shone laser light through them and calculated the speckle size(experiment done for concentration assessment).The result I obtained showed speckle size decreasing with increase in concentration.Can anyone give me a logical explanation as to why this is.

I'm a little confused, since light scattering measurements typically measure time-rates of change of the signal at a point as opposed to imaging the speckle. What is/are your bead concentration(s)? Speckle size (edit: more correctly, the coherence volume) should be set by the source, not the object. Perhaps, if you have multiple scattering the speckle pattern could be disrupted.

 

[Daymare]

The bead concentrations are of the order of 10^7 particles per milli liter.

Speckle size should be set by the source,

It depends on the source,the aperture and how far the detector is kept right?Well DLS experiments typically measures times rates of the signal,but here I am doing a very easy experiment and I am just using it to measure the speckle size.After getting the speckle size I am just plotting the speckle size with concentration to see how it goes.I am just trying to do things by my own here,reading papers from here and there.And I saw that most guys keep detector at an angle(I think they do this as, if its kept head on,the intensity of the laser would be too much to observe any relevant speckle patterns)
After doing the experiment I saw that speckle size seem to decrease with concentration.This is opposite to what comes to us naturally(more concentration less is the aperture space for light to go through and hence larger should the speckle size be).I am assuming this has to something to do with Mei's scattering which I have no clue.What I am looking for is why this is so.
Also correct me if I have done the experiment incorrectly,I also don't have much idea on speckle's.

 

[Andy Resnick]

The bead concentrations are of the order of 10^7 particles per milli liter.

That comes out to 0.1% volume fraction, which seems a little high- you only want single scattering, not multiple scattering. Try diluting 1:1000 and see if you still have problems.

It depends on the source,the aperture and how far the detector is kept right?

Yes, in a manner of speaking. The coherence volume is set by the angular size of the source and bandwidth. Assuming the aperture is uniformly illuminated, the angular size is set by the aperture diameter and the distance between the aperture and sample, because the aperture is the source.

Well DLS experiments typically measures times rates of the signal,but here I am doing a very easy experiment and I am just using it to measure the speckle size.After getting the speckle size I am just plotting the speckle size with concentration to see how it goes.I am just trying to do things by my own here,reading papers from here and there.

The speckle size should be independent of the sample concentration *if* there is only single scattering processes going on. As for papers, Dave Weitz has some nice ones on diffusing wave spectroscopy, which is relevant for turbid samples.

And I saw that most guys keep detector at an angle(I think they do this as, if its kept head on,the intensity of the laser would be too much to observe any relevant speckle patterns)

Low-angle scattering is problematic for that reason- but low angle scattering also probes the slowest time scales, so getting that information is important for many studies of dynamic systems. Often, people choose a single angle to simplify the setup, but there's at least one approach that can (in principle) obtain DLS data over many angles simultaneously, by performing the measurement with a microscope. The objective collects light over many scattering angles, and imaging the back pupil plane rather than the object plane (with a Bertrand lens, or however else is convenient) then associates each pixel of a sensor with a particular scattering angle; rapid frame rates then enable time-series measurements.

After doing the experiment I saw that speckle size seem to decrease with concentration.This is opposite to what comes to us naturally(more concentration less is the aperture space for light to go through and hence larger should the speckle size be).I am assuming this has to something to do with Mei's scattering which I have no clue.What I am looking for is why this is so.

My guess is that you have multiple scattering- the scattering decreases the coherence, resulting in smaller speckles. A good introduction to coherence is Wolf "Introduction to the theory of coherence and polarization of light", unfortunately, I don't have a decent reference on DLS to offer.

 

[Daymare]

Thank you,that was helpful.I will look into what you have said and see what I can figure out.