Tunable viscosity modification with diluted particles: When particles decrease the viscosity of complex fluids

While spherical particles are the most studied viscosity modifiers, they are well known only to increase viscosities, in particular at low concentrations. Extended studies and theories on non-spherical particles find a more complicated behavior, but still a steady increase. Involving platelets in combination with complex fluids displays an even more complex scenario that we analyze experimentally and theoretically as a function of platelet diameter, to find the underlying concepts. Using a broad toolbox of different techniques we were able to decrease the viscosity of crude oils although solid particles were added. This apparent contradiction could lead to a wider range of applications.Comment: 13+7 pages, 6+7 figure

Capillary condensation in microemulsions

Microemulsions are thermodynamically stable mixtures of oil and water that are mediated by the surfactant. Locally, there are oil and water domains that are observable by scattering experiments that are separated by the surfactant film. The domain sizes are usually a few nanometers and display shapes from spherical droplets over elongated droplets to the bicontinuous sponge phase. In the following we restrict ourselves to the bicontinuous microemulsion. When exposing the microemulsion to hydrophilic surfaces, a lamellar order is locally induced next to the interface. From spectroscopic measurements, we know that the membrane fluctuations in microemulsions are faster in the lamellar state. This is connected to the lubrication effect, because the lamellae can slide off easier and the motions are faster. In spectroscopic measurements with hydrophilic clay particles, we could show, that the platelet diameter causes a cutoff of the undulation modes, and larger platelets cause a better order with longer wavelength modes. The capillary condensation in bicontinuous microemulsions is expected to take place when two parallel surfaces are narrowed that result in a completely lamellar microemulsion (Figure). So far, all experiments stayed at relatively low clay concentrations, when the system is still liquid. The lamellar fraction in microemulsions with 1% clay is around 25% in volume. We now tried to observe the capillary condensation with increasing clay concentration using small angle neutron scattering (SANS) and neutron spin echo (NSE) spectroscopy

Capillary condensation and gelling of microemulsions with clay additives

The capillary condensation in bicontinuous microemulsions takes place when two parallel surfaces are narrowed that result in a completely lamellar microemulsion. We expected that this phase transition is also observable when the amount of hydrophilic surfaces from clay particles is raised, because hydrophilic surfaces induce lamellar ordering locally. Using small angle neutron scattering, the structure of micreoemulsions was observed as a function of clay content. The critical concentration is indicated by discontinuous structural changes and depends on the platelet diameter and is explained by the free energy of the platelets competing with the fluctuating medium. The gel phase transition is observed in the spectroscopic measurements where the diffusion motion is widely suppressed in the gel phase, but otherwise superimposes with the membrane undulations

Viscoelastic membrane modes adjacent to a hard wall

SoyPC membrane stack modes adjacent to a hard hydrophilic wall were observed to be of viscoelastic character on nanosecond time scales [1]. This is astonishing because nearly all soft matter systems display over-damped behavior on nanosecond time scales, because the molecular friction is rather strong. This has direct impact on energy dissipation along the membrane, because shock energies could be distributed over larger areas as classically into the volume in the normal direction. Membrane stacks in mammalian joints could benefit from that effect.We started to observe soyPC membrane stacks in terms of structure using grazing incidence small angle neutron scattering. These systems we doped with ibuprofen [2] molecules to observe the malfunction of the membrane. Ibuprofen is known to cause stomach bleeding upon extensive use.The pure membranes in water were studied using grazing incidence neutron spin echo spectroscopy. One drawback of this method is the weak scattering intensity resulting from small scattering volumes next to the solid-liquid interface and from the small apertures that are needed to define the incident angle well. A neutron resonator [3] was developed to enhance the impinging wave-field dramatically such that neutron spectroscopy at interfaces becomes feasible.References[1] S. Jaksch, O. Holderer, M. Gvaramia, M. Ohl, M. Monkenbusch, H. Frielinghaus, Sci. Reports 7, 4417 (2017)[2] S. Jaksch, F. Lipfert, A. Koutsioubas, S. Mattauch, O. Holderer, O. Ivanova, H. Frielinghaus, S. Hertrich, S.F. Fischer, B. Nickel, Phys. Rev. E 91, 022716 (2015)[3] H. Frielinghaus, M. Gvaramia, G. Mangiapia, S. Jaksch, M. Ganeva, A. Koutsioubas, S. Mattauch, M. Ohl, M. Monkenbusch, O. Holderer, Nucl. Instr. Meth. Phys. Res. A 871, 72-76 (2017

Neutron spin echo spectroscopy with a moving sample

Abstract Neutron spin echo spectroscopy is a high resolution inelastic neutron scattering method probing nanosecond dynamics. It is well suited to study the atomistic motion in polymer systems and contributes to our understanding of viscoelasticity. However, for samples under shear, or moving samples in general, Doppler scattering has to be considered. We compare the measured phase shift and depolarisation due to Doppler scattering from a rotating graphite disk to numerical and analytical calculations and find excellent agreement. This allows to take into account Doppler scattering during the data processing and makes longer Fourier times as well as higher shear rates and Q ranges possible with neutron spin echo spectroscopy, enabling for example the study of polymers under high shear

Nanoscale rheology at solid-complex fluid interfaces

Here we present an approach to measure dynamic membrane properties of phospholipid membranes close to an interface. As an example we show results of the membrane dynamics of a phospholipid membrane multilayer-stack on a solid substrate (silicon). On this sample we were able to measure local interaction and friction parameters using Grazing Incidence Neutron Spin Echo Spectroscopy (GINSES), where an evanescent neutron wave probes the fluctuations close to a rigid interface. With this method it is possible to access length scales in the nano to micrometer region as well as energies in the μeV range. Using a new neutron resonator structure we achieved the required intensity gain for this experiment. During our investigations we found an excitation mode of the phospholipid membrane that has not been reported previously and only became visible using the new methodology. We speculate that the energy transported by that undulation can also serve to distribute energy over a larger area of the membrane, stabilizing it. This new methodology has the capability to probe the viscoelastic effects of biological membranes, becoming a new tool for tribology on the nanoscale and has allowed the observation of the hitherto invisible property of phospholipid membranes using neutrons

Thickness of the particle-free layer near charged interfaces in suspensions of like-charged nanoparticles

When a suspension of charged nanoparticles is in contact with a like-charged water–solid interface, next to this interface a particle-free layer is formed. The present study provides reliable measurements of the thickness of this particle-free layer with three different techniques, namely optical reflectivity, quartz crystal microbalance (QCM), and direct force measurements with atomic force microscopy (AFM). Suspensions of negatively charged nanoparticles of different size and type are investigated. When the measured layer thickness is normalized to the particle size, one finds that this normalized thickness shows universal inverse square root dependence on the particle volume fraction. This universal dependence can be also derived from Poisson–Boltzmann theory for highly asymmetric electrolytes, whereby one has to assume that the nanoparticles represent the multivalent coions

Effect of Benzocaine and Propranolol on phospholipid-based bilayers

Cell membranes play a fundamental role in protecting the cell from its surroundings, in addition to host many proteins with fundamental biological tasks. A study of drug/lipid interactions is a necessary and important step to fully clarify the role and action mechanism of active ingredients, as well as to shed light on possible complications caused by drug overdosage. In this paper, the influence of Benzocaine and Propranolol drugs on the structure of L-α-phosphatidylcholines-based membranes has been investigated by means of neutron reflectivity, grazing incidence small angle neutron scattering, and small/ultrasmall angle neutron scattering. Investigations allowed discovering a stiffening of the membranes as well as the formation of stalks, caused by the presence of Benzocaine. On the other hand, disordered bilayers (lamellar powders) and highly curved structures were found in the presence of Propranolol. The found results may be rationalized in terms of the molecular structure of drugs and may serve as starting point for explaining the toxic behavior in long-term and overdosage scenarios

Simpler neutron resonator enhances the wave-field for grazing incidence scattering experiments with lower parasitic scattering

A simpler resonator structure with less layers has been tested with respect to the impinging wave enhancement in terms of amplified scattering signals. The results are compared with our first resonator with three double layers in terms of background signal and intensity gain. The new resonator promises much better performance than the earlier version