I will try to make is simple here in addition to the information given by the previous replies.

You are using DLS when you want to determine your particle size based on dynamic brownian motion of your suspended particle in whatever medium you are using (namely water or other liquid). Mostly, you will have your particle hydrodynamic diameter or radius. Basically your particle size is determined using D=kT/[6*pi*neta*Rh) where D is diffusion coefficient, k=boltzmann's constant, T=absolute temperature, neta=viscosity of your dispersing medium and Rh is your hydrodynamic radius. Your instrument (DLS) typically using HeNe laser and the scattering signals being picked up at 90 degree angle or in the case on Malvern (as mentioned previously) may be measured through back scattering detector so that it can also measure high concentration or highly turbid (~30% I think). There are other parameters required prior to measurement. you can read more details in many reliable websites. What you will obtain eventually is, distribution of your particle size in the solution, or Z-average in the case of Malvern.

Some instrument can measure both Zeta and size. In the case of malvern zeta sizer (just example because it was mentioned earlier) you may use the same u-tube cuvette for both measurements. Remember they are different technique altogether.

Zeta potential is only an estimation of surface charge and not an absolute value. Approximate value at stern layer, one-two counter ion distance from particle surface. Thus sometimes you may need to add electrolyte in your solution. The value that normally determined by many instruments is, electrophoretic mobility of your particle as it moves across from one electrode to the other. Doesn't matter anode to cathode or vice versa depending on positive or negative surface charge.

So you have two electrodes with potential value between them as (V), knowing the distance between them, may keep the current constant. As you increase your voltage across the two electrodes, you will observed and increase in the particle velocity. Some using doppler effect (you need a laser too). Knowing your voltage/electrode distance, velocity, may use Henry's F(a) factor = 1, thus zeta potential can be derived.

Rule of thumb, zeta values must be greater than +/- 30 mV for stable suspension which may be true for sols.

You have some machine using DLS to determine the size of the particles, zeta potential is linked to the surface charge of your particles. both things are different but some machine using different algorythms can be used to determine size & zeta potential, this is for exmaple the case of the zeta sizer from Malvern.

Dear fatima

Well DLS is used to gather the knowledge of the nanoparticles in respect to their hydrdynamic diameter whereas zeta potential is used to know the surface charge variation and colloidal stability depending on various pH range from pH 2 to pH 10.

i hope your doubt is clear now.

Thanks

So DLS is able to measure both zeta potential and hydrodynamic diameter? I understand that they are both different things but they rely on the same technique, i.e DLS to obtain the retrospective results?

Many thanks for both your responses.

I will try to make is simple here in addition to the information given by the previous replies.

You are using DLS when you want to determine your particle size based on dynamic brownian motion of your suspended particle in whatever medium you are using (namely water or other liquid). Mostly, you will have your particle hydrodynamic diameter or radius. Basically your particle size is determined using D=kT/[6*pi*neta*Rh) where D is diffusion coefficient, k=boltzmann's constant, T=absolute temperature, neta=viscosity of your dispersing medium and Rh is your hydrodynamic radius. Your instrument (DLS) typically using HeNe laser and the scattering signals being picked up at 90 degree angle or in the case on Malvern (as mentioned previously) may be measured through back scattering detector so that it can also measure high concentration or highly turbid (~30% I think). There are other parameters required prior to measurement. you can read more details in many reliable websites. What you will obtain eventually is, distribution of your particle size in the solution, or Z-average in the case of Malvern.

Some instrument can measure both Zeta and size. In the case of malvern zeta sizer (just example because it was mentioned earlier) you may use the same u-tube cuvette for both measurements. Remember they are different technique altogether.

Zeta potential is only an estimation of surface charge and not an absolute value. Approximate value at stern layer, one-two counter ion distance from particle surface. Thus sometimes you may need to add electrolyte in your solution. The value that normally determined by many instruments is, electrophoretic mobility of your particle as it moves across from one electrode to the other. Doesn't matter anode to cathode or vice versa depending on positive or negative surface charge.

So you have two electrodes with potential value between them as (V), knowing the distance between them, may keep the current constant. As you increase your voltage across the two electrodes, you will observed and increase in the particle velocity. Some using doppler effect (you need a laser too). Knowing your voltage/electrode distance, velocity, may use Henry's F(a) factor = 1, thus zeta potential can be derived.

Rule of thumb, zeta values must be greater than +/- 30 mV for stable suspension which may be true for sols.

Both DLS and zeta-potential can be obtained from light scattering. While zeta can be obtained from doppler effect as described in Misni Misra's answer, the mean hydrodynamic diameter is obtained from the correlation between the intensity of scattered light and time. This correlation is used for the brownian motion analysis also described above by Misni.

Dynamic light scattering (DLS) measures the intensity fluctuations of light scattered from randomly moving, diffusing particles. From these fluctuations the diffusion coefficient and thus the size can be obtained.

Electrophoretic light scattering (ELS) measures the intensity of light scattered from particles moving in an applied electric field. From these intensity changes the electrophoretic mobility (due to the movement in the electric field on top of the diffusion) and then the zeta potential can be obtained.

http://www.materials-talks.com/blog/2016/07/21/dynamic-light-scattering-primer-4m22s/