Masoud Amirkhani

The effect of simultaneous size reduction and transient network formation on the dynamics of microemulsions

We studied a mixture of C12E5 microemulsion with an end-capped hydrophobically modified polymer (C12–PEO–C12). The end-capped polymer adsorbed on the core of the microemulsion, changed the bending properties of the interface and connected two nearby droplets. The core size and width of the structure factor of the microemulsion decreased upon adding the end-capped polymer. For all polymer concentrations, two relaxational modes corresponding to different diffusion coefficients were observed. The fast and slow diffusion coefficients showed a repulsive and an attractive interaction, respectively. The results of both small-angle x-ray scattering and dynamic light scattering suggested that fast relaxation is affected by size reduction and increasing the repulsive interaction between droplets. (Some figures may appear in colour only in the online journal)

The effect of different polymer length on water droplets of reverse AOT microemulsion

We study the effect of polyethylene glycol (PEG) on the dynamic and structure of water droplets at the reverse sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) microemulsion. The mixture of water and oil with anionic surfactant AOT can form microemulsion. The dynamic of microemulsion in the presence of PEG is investigated by photon correlation spectroscopy technique. We mainly focus on the variation of the translational diffusion behaviour as a function of the polymer concentration and polymer length scale. By increasing the content of the lowest PEG length scale (Mn = 285), the dynamic of microemulsion slows down. In addition, one relaxation process is distinguished for all polymer concentration. However, for the two higher polymer length scale (Mn = 2200 and 6000), two relaxations are observed and the dynamic of microemulsion speeds up. We used the small angle X-ray scattering technique to monitor the size and the polydispersity of the mixture system (AOT microemulsion/PEG).

The effect of TBAC on the collective diffusion coefficient and morphology of AOT microemulsion at X = 6.7

This study investigates water-in-oil microemulsion stabilised by Aerosol OT (AOT) dispersed in n-decane at water/AOT molar ratio of 6.7. The collective diffusion coefficient of microemulsion and microemulsion/tetrabutylammonium chloride (TBAC) systems was investigated by photon correlation spectroscopy (PCS). In the systems at the constant water/AOT concentration, the collective diffusion decreases with the reduction of oil concentration. Furthermore, PCS experiment revealed that collective diffusion coefficient of AOT/H2O/n-decane microemulsions increased with TBAC added to the microemulsion. We also studied the structural information of pure and salt-mixed AOT microemulsion by small-angle X-ray scattering technique. The structural investigation of our samples shows cylindrical to spherical transition with the reduction of oil concentration.

Depletion-induced sphere-cylinder transition in C12E5 microemulsion: a small-angle X-ray scattering study

Small-angle X-ray scattering was used to study the mixture of C12E5 (pentaethylene glycol monododecyl ether)/H2O/n-decane microemulsion and polyethylene glycol (PEG). The size, shape and the structure factor of the microemulsion were investigated by adding the polymer (PEG) to the mixture. Attractive depletion potential was induced between the microemulsion droplets by the non-adsorb polymer. The range and strength of the attractive potential were changed by varying the molecular weight and concentration of PEG. The forward scattering, S(0), of the spherical microemulsion, declined gradually as the polymer concentration decreased. For PEG with the molecular weight of Mn = 285−315, the microemulsion morphology remained spherical, but the main peak of the structure factor moved towards a bigger q. When PEG with molecular weights of Mn = 2200 and Mn = 6000 were used, a shape transition from spherical to cylindrical was induced in line with increasing polymer concentration.

The mixture of poly(propylene-glycol)-block-poly(ethylene-glycol)-block-PPG with C12E5 microemulsion

We study oil-in-water microemulsions, in particular dispersions of n-decane droplets coated with a monolayer of the surfactant C12E5 in a continuous phase of water. Upon addition of the tri-block poly(propylene-glycol)(PPG)-block-Poly(ethylene-glycol)(PEG)-block-PPG, the depletion interaction between the oil droplets of microemulsion is induced. At constant droplet size, we vary the polymer concentration and there is clear evidence for an increasing depletion interaction of the droplets from structural investigations with small-angle x-ray scattering and photon correlation spectroscopy (PCS). Our results show that the PEG does increase the depletion interaction more than the tri-block PPG-PEG-PPG.

An external disturbance sensor for ionic polymer metal composite actuators

Ionic polymer metal composite (IPMC) is a fast-growing type of smart material with a wide range of applications. IPMC has been used extensively as an actuator, but for effective usage, one must add a self-sensing ability to it. Two common self-sensing techniques are mechanical-to-electrical transducer and surface resistance. The first one cannot be used while the actuator is running, and the second one needs a sample modification. In this work, we present a new self-sensing method, which can measure external disturbance in the presence of actuator voltage without any sample modification. The resistance across an IPMC sample follows Ohms law at sufficiently high frequency. We exploit the frequency dependency of the resistance across the sample to design the self-sensing method. In this technique a function generator, a lock-in amplifier and an isolation circuit were employed to measure an external impulse or steady disturbance. As implementing this technique does not require any change to the IPMC specimen or electrical connection (hanger), it can be added to any existing electroactive device.

Measurement of contact angles of microscopic droplets by focal length method

We present a method to measure contact angles of microscopic droplets with a conventional microscope that possesses a precision focus adjustment stage. The droplets are modeled as spherical caps that act as lenses. Their focal length is determined by measuring the distance from the substrate surface to the level where a sharp image of the aperture stop is observed. The lens diameter is found by edge detection of a microscope image of the microdroplets. The spherical cap model relates the focal length and diameter of such lenses to the contact angle of the used liquid with known refractive index. The measurement procedure was applied to condensed water droplets on a silicon substrate covered by its native oxide layer. The results are found to be in good agreement with conventional, goniometric sessile drop measurements of the advancing contact angle.

Conformation and structural changes of diblock copolymers with octopus-like micelle formation in the presence of external stimuli

External stimuli such as vapours and electric fields can be used to manipulate the formation of AB-diblock copolymers on surfaces. We study the conformational variation of PS-b-PMMA (polystyrene-block-poly(methyl methacrylate)), PS and PMMA adsorbed on mica and their response to saturated water or chloroform atmospheres. Using specimens with only partial polymer coverage, new unanticipated effects were observed. Water vapour, a non-solvent for all three polymers, was found to cause high surface mobility. In contrast, chloroform vapour (a solvent for all three polymers) proved to be less efficient. Furthermore, the influence of an additional applied electric field was investigated. A dc field oriented parallel to the sample surface induces the formation of polymer islands which assemble into wormlike chains. Moreover, PS-b-PMMA forms octopus-like micelles (OLMs) on mica. Under the external stimuli mentioned above, the wormlike formations of OLMs are able to align in the direction of the external electric field. In the absence of an electric field, the OLMs disaggregate and exhibit phase separated structures under chloroform vapour.