Final Report Summary - DYNEFI (Dynamic Near Field Imaging)
Project context
Dynamic Light Scattering is a popular technique for measuring fluid properties in soft matter, fluid physics, and biophysics. It is used to investigate complex fluids, colloids, proteins, nanoparticles, and also samples of biophysical and medical interest. The advent of pixilated sensors like CCD cameras and fast computers allowed the design of imaging techniques aimed at providing the same quantitative information. Dynamic Near Field Imaging (DyNeFI) is a family of novel optical techniques of this kind. Different variants of this new approach have been successfully designed and used for very refined measurements. Nevertheless, the scientific community is not fully aware of this new intriguing possibility.
Project objectives
The proposed project aims to increase the measuring capabilities of DyNeFI by integrating new measuring concepts, applying it to samples coming from different research areas, thereby providing new science and starting developing instruments which can be commercialised in the near future.
Project results
The activities performed have demonstrated that the DyNeFI is a mature approach for investigating many different kinds of samples. In the Soft Matter and Photonics Group at the University of Fribourg in Switzerland we have developed new software by taking advantage of the capabilities offered by modern graphics cards to perform parallel calculations. This new software made it possible to perform differential dynamic analysis in a time comparable to the time needed to acquire the images, making the analysis quasi-real-time.
We have performed refined measurements on the concentration non-equilibrium fluctuations in a binary liquid mixture under thermal stress. Even if a fluid exhibits fluctuations of its thermodynamic variables in the equilibrium state, too, non-equilibrium conditions greatly enhance them so that they became detectable mainly at the mesoscale. The theory predicts divergence at large scales, which is prevented only by the presence of gravity acting for fluctuations larger than a critical size. We have demonstrated that measuring the value of this critical size can provide a precise measurement of the Soret coefficient of a binary mixture. Moreover, from the evaluation of the relaxation constants as a function of the scattering vector, the mass diffusion coefficient can be derived. Therefore, the analysis of non-equilibrium fluctuations by means of DyNeFI technique can provide a robust and accurate measurement technique for these two important transport properties of fluid mixtures.
In a similar way, the analysis has been applied to confocal images of droplets of an oil-in-water emulsion, providing an accurate measurement of the system dynamics and, at the end, the size of the nanoscale droplets. The shadow-graph technique is historically widely utilised for visualising large-scale refractive index variations within a fluid. Therefore, we used our set-up to investigate the convective instabilities generated by the Soret separation in a binary mixture with positive Soret coefficient heated from below. In these conditions the fluid is unstable because of the density gradient given by the thermal one, but the Rayleigh number associated to the thermal contribution is not sufficient to promote convection. After a diffusion time, a concentration gradient sets in due to the Soret effect and, in this case, the density mismatch is enough to start convection. We focused our attention on the effect of a small tilt of the sample, with regard to the gravity direction, on the stability of the convective patterns.
This study was initiated as a lateral project, but the results have been so surprising and interesting that we have decided to spend the last six months of the project on this topic. We have observed that even a marginal inclination can strongly affect the stability of the convective patterns in the long term. At small Rayleigh numbers the inclination gives rise to a large-scale circulation within the cell, which destroys the small-scale convective patterns. On the contrary, for larger Rayleigh numbers, the columnar flow typical of Rayleigh-Bénard convection remains the most efficient convection mechanism. At the limit between these two behaviours a new convective state has been detected which we called super highway convection, because the columns of fluid organise in parallel lines moving in opposite directions in a fashion reminiscent of cars moving in different lanes on a super-highway.
This work can open new possibilities for the study of other systems in which concentration and temperature profiles are involved, like the thermohaline convection in the oceans, a phenomenon which is still not completely understood. This part of the work was not directly within the scope of the original proposal, but it perfectly suits its main objective which is to apply the DyNeFI technique to different physical problems to investigate new science and make the widest possible scientific community aware of the capabilities of the DyNeFI technique.
Dynamic Light Scattering is a popular technique for measuring fluid properties in soft matter, fluid physics, and biophysics. It is used to investigate complex fluids, colloids, proteins, nanoparticles, and also samples of biophysical and medical interest. The advent of pixilated sensors like CCD cameras and fast computers allowed the design of imaging techniques aimed at providing the same quantitative information. Dynamic Near Field Imaging (DyNeFI) is a family of novel optical techniques of this kind. Different variants of this new approach have been successfully designed and used for very refined measurements. Nevertheless, the scientific community is not fully aware of this new intriguing possibility.
Project objectives
The proposed project aims to increase the measuring capabilities of DyNeFI by integrating new measuring concepts, applying it to samples coming from different research areas, thereby providing new science and starting developing instruments which can be commercialised in the near future.
Project results
The activities performed have demonstrated that the DyNeFI is a mature approach for investigating many different kinds of samples. In the Soft Matter and Photonics Group at the University of Fribourg in Switzerland we have developed new software by taking advantage of the capabilities offered by modern graphics cards to perform parallel calculations. This new software made it possible to perform differential dynamic analysis in a time comparable to the time needed to acquire the images, making the analysis quasi-real-time.
We have performed refined measurements on the concentration non-equilibrium fluctuations in a binary liquid mixture under thermal stress. Even if a fluid exhibits fluctuations of its thermodynamic variables in the equilibrium state, too, non-equilibrium conditions greatly enhance them so that they became detectable mainly at the mesoscale. The theory predicts divergence at large scales, which is prevented only by the presence of gravity acting for fluctuations larger than a critical size. We have demonstrated that measuring the value of this critical size can provide a precise measurement of the Soret coefficient of a binary mixture. Moreover, from the evaluation of the relaxation constants as a function of the scattering vector, the mass diffusion coefficient can be derived. Therefore, the analysis of non-equilibrium fluctuations by means of DyNeFI technique can provide a robust and accurate measurement technique for these two important transport properties of fluid mixtures.
In a similar way, the analysis has been applied to confocal images of droplets of an oil-in-water emulsion, providing an accurate measurement of the system dynamics and, at the end, the size of the nanoscale droplets. The shadow-graph technique is historically widely utilised for visualising large-scale refractive index variations within a fluid. Therefore, we used our set-up to investigate the convective instabilities generated by the Soret separation in a binary mixture with positive Soret coefficient heated from below. In these conditions the fluid is unstable because of the density gradient given by the thermal one, but the Rayleigh number associated to the thermal contribution is not sufficient to promote convection. After a diffusion time, a concentration gradient sets in due to the Soret effect and, in this case, the density mismatch is enough to start convection. We focused our attention on the effect of a small tilt of the sample, with regard to the gravity direction, on the stability of the convective patterns.
This study was initiated as a lateral project, but the results have been so surprising and interesting that we have decided to spend the last six months of the project on this topic. We have observed that even a marginal inclination can strongly affect the stability of the convective patterns in the long term. At small Rayleigh numbers the inclination gives rise to a large-scale circulation within the cell, which destroys the small-scale convective patterns. On the contrary, for larger Rayleigh numbers, the columnar flow typical of Rayleigh-Bénard convection remains the most efficient convection mechanism. At the limit between these two behaviours a new convective state has been detected which we called super highway convection, because the columns of fluid organise in parallel lines moving in opposite directions in a fashion reminiscent of cars moving in different lanes on a super-highway.
This work can open new possibilities for the study of other systems in which concentration and temperature profiles are involved, like the thermohaline convection in the oceans, a phenomenon which is still not completely understood. This part of the work was not directly within the scope of the original proposal, but it perfectly suits its main objective which is to apply the DyNeFI technique to different physical problems to investigate new science and make the widest possible scientific community aware of the capabilities of the DyNeFI technique.