Interactions between colloidal particles at oil-water interfaces

The behavior of colloidal particles at two-dimensional interfaces is of considerable interests in terms of industrial applications and model systems for research. To systematically study colloidal particle behaviors at 2D interfaces, we directly measure interaction forces between particle pairs, dynamics of a confined particle in a fixed geometry, and micromechanical properties of aggregates or percolated networks. For doing this, we use multiple time-shared optical traps which enable us to measure forces in the piconewton range (0.1-100 pN).;First, the trapping forces for a single ray at the interface were calculated while varying the particle position at the 2D interfaces. The trapping force is described by the contributions of reflected and refracted rays with their corresponding powers, which are determined by Fresnel reflection and transmission coefficients. Based on the Ashkin's calculation in the 3D aqueous phase, we found that the trapping and scattering forces are mainly determined by the first reflection of the incident ray and the next three refractions to the medium. At 2D interfaces, the trapping force is considered for two geometries; (a) when the incident ray is incident on a particle in the aqueous phase, (b) when a transmitted ray at the liquid interface enters into a particle in the oil phase. In the first geometry, the trapping forces do not change, regardless of the refraction orders if the second refracted ray is to the oil phase. The dimensionless factors Qg and Qs of the gradient force and the scattering force decrease compared to those in the 3D aqueous space, and Qs decreases more than Qg. This suggests that the presence of the interface provides good trapping conditions. In the second geometry, the magnitude of dimensionless factors is consistent with those calculated in the first geometry when the incident angle is in the range of 0<theta3<70°. Above 70° of the incident angle, the refraction to the aqueous phase does not occur, suggesting the total internal reflection inside the particle. If the first four refractions to the medium take place only in the oil phase, which is the same condition with the case in the 3D oil phase, the values of Q are slightly lower than those in the aqueous phase.;Second, we directly measured the pair interaction forces at the oil-water interface by manipulating the subphase conditions via the introduction of salt and surfactant. For the subphase containing a small amount of salt or surfactant, as well as a virgin system, the long-range repulsive force profiles scale as r-4, which is consistent with dipole interactions. Due to heterogeneities in the pair interaction forces, measurements are averaged over many particle pairs to obtain suitable trends. The interaction forces are measured as a function of the time to evaluate the time evolution, and are characterized by the magnitude of repulsive force F 0. For salt in the subphase, we found that the average values of F0 extrapolated to t=0 are weakly dependent on the Debye screening length. This is accounted by considering charge renormalization when the particles possess high surface charge. However, the magnitude of the measured repulsive force is larger than the predicted when the charges in the aqueous phase are only considered. To examine the charge effects in the oil side, we introduced SDS in the subphase, which changes the wetting properties, and increases the three phase contact angle pushing the particle further into the oil. The values of F0 obtained in the SDS experiments show similar behavior to the salt experiments when SDS is treated as a 1:1 electrolyte. This implies that the charges in the aqueous phase play a dominant role in the long-range repulsive interactions. Interestingly, we observed the time evolution of the repulsive force for SDS subphase conditions. This is significant because it provides a possible mechanism for the reported flocculation behavior of 2D suspensions. Finally, we found that the long-range attractive interaction arises, showing a secondary minimum when the surface charges in the aqueous phase are screened by a large number of counterions.