Project

Advanced optical trapping electrophoresis of single colloidal particles

Code
178TW1014
Duration
01 January 2014 → 31 December 2017
Funding
Regional and community funding: IWT/VLAIO
Research disciplines
  • Engineering and technology
    • Electronics
Keywords
optical trapping electrophoresis colloidal suspension
 
Project description

A colloid or a colloidal suspension is a mixture in which microscopic or nanoscopic particles are dispresed throughout a liquid medium. Milk is a colloidal suspension of fat droplets in water and printer ink is a colloidal suspension ofsolid ink particles in oil. An important characteristic of colloidal
suspensions is its stability. The stability of colloidal suspensions is hindered by aggregation and sedimentation. This occurs when attractive interparticle forces prevail over repulsive interparticle forces. A common way to prevent
aggregation and sedimentation is to increase repulsive forces by increasing the particle’s electric charge. Coulombic forces then prevent particles from making contact with each other or with the substrate interface.
Electric phenomena form a fundamental aspect of colloidal suspensions and they are well-understood in aqueous suspensions. The same is not true for colloidal suspensions in oily liquids. A key difference between oils and water
lies in their respective dielectric constants. Due to its high relative dielectric constant (2r,H O = 80), water provides a suitable host medium for ionic molecules where they can be freely dispersed. Oily liquids such as ndodecane (12 26 r C H ,
= 2) on the other hand do not. Liquids with high relative
dielectric constants, such as water, are called polar liquids and liquids with low dielectric constants, such as dodecane, are called nonpolar liquids. Due to the low relative dielectric constant, electrostatic interactions in nonpolar liquids are long ranged when compared to water, which is why oppositely
charged ions are more likely to neutralize each other. Colloidal particles in pure nonpolar liquids commonly are not sufficiently charged to repel each other, causing aggregation or sedimentation.
There exist chemical compounds that, when added to a nonpolar colloidal suspension, cause the colloidal particles to become highly charged. These chemical compounds are called surfactants. Surfactant molecules typically
consist of polar head groups with long nonpolar tails. Above a certain concentration, surfactants in nonpolar liquids form spherical structures, called inverse micelles, where the polar head groups of the molecules are pointed inwards and the nonpolar tails are pointed toward the liquid medium. This concentration is called the critical micelle concentration.
The mechanism through which particles in nonpolar liquids become charged by surfactants however, is still the subject of much debate and scientific research. Different possible surfactant related charging mechanisms have been put forward and in several specific particle-liquid-surfactant combinations it has been determined what the dominant charging mechanism is. However, to date, it is often still impossible to state in advance what the dominant charging mechanism will be for a colloidal system. This is partly due to the complexity of surfactant-enriched colloidal nonpolar systems, but also partly due to the limited scope of currently available measurement techniques.
Optical trapping electrophoresis remains the only experimental technique that has demonstrated the capabilty of measuring the charge of colloidal particles with such precision that single elementary charging and discharging events can be detected and that can do so without time restrictions. The ability to resolve elementary (dis)charging events on a single particle opens up new avenues to study colloidal charging phenomena. In previous work, the detection of elementary charging events on single particles has been achieved for particles in a pure nonpolar liquid. The aim of this work is to improve optical trapping electrophoresis as a technique, so that single
charging events can be detected on particles in nonpolar liquids with surfactants. An important milestone is the detection of elementary charging events at surfactant concentrations above the critical micelle concentration.
In order to achieve that goal, all stages that together make up optical trapping electrophoresis are revisited. I redesigned the microfluidic cell in which the colloids are examined. The cell consists of two parallel glass plates that are covered with a thin ITO electrode. The electric field is now perpendicular to the liquid-solid interface of the cell. This has the advantage
that liquid flows generated by the presence of an electric field are prevented.
Another advantage is that the cells are now created under cleanroom conditions to prevent contamination by undetermined species. I also put forward a protocol to align the laser prior to each measurement. Finally, I extended the analysis method that is used to resolve elementary charging
events from the measurement data.
First, I studied micrometer sized PHSA-coated PMMA particles in dodecane without surfactant. Experiments revealed that the particle’s charge was not stable over time. Instead, particles accumulated positive surface charge over
time. Confronted by this unanticipated phenomenon, I investigated how the particle’s charging rate changed when factors such as the amplitude and the frequency of an AC electric field, the particle’s diameter, the electrode
material, the absence of an electric field and the presence of a DC electric field are changed. I also studied the long-term saturation of this effect.
Through the ensemble of these experiments, I concluded that these particles shed negatively charged ions under the influence of an electric field.
Then, I studied how the charging mechanism changes when surfactant is introduced. In this work, we used OLOA 11000 as surfactant. By applying a DC voltage of 1 V, the concentration range in which the particle’s charge can be measured with a charge resolution smaller than the elementary charge
was extended to 0.05 wt% OLOA in dodecane, which encompasses the critical micelle concentration of OLOA 11000 in dodecane. This marks the first experimental demonstration of elementary event monitoring in surfactant-enriched environments. One contributing reason why a DC
voltage helps increase this range, is because it depletes the bulk of the liquid from ionic species. The presence of ions impedes the oscillatory motion of a particle in an AC electric field through the retardation effect. Under these conditions, I showed that the particle’s charging frequency increases as the
surfactant concentration increases. Around the critical micelle
concentration, the particle’s charge stabilizes and fluctuates around a mean charge value.
In summary, in this work, I extended the concentration range in which optical trapping electrophoresis can be used to monitor a particle’s charge with a charge resolution smaller than the elementary charge and thus to detect elementary charging events. I demonstrated how this can be used to gain
new insights in the charging mechanisms of colloidal particles both in pure nonpolar liquids and in surfactant-enriched nonpolar systems at concentrations above the critical micelle concentration