Schematics of the interface shear micro-rheometer, the cell used for a sample and an example of a film with an optically trapped particle at the air/water interface. (a) Green: Nd:YAG laser, λ = 532 nm, IR: Nd:YAG laser, λ = 1064 nm, DM: dichroic mirror, OBJ: objective lens, CM: cold mirror, QPD1, QPD2: quadrant photodiode. A lock-in amplifier and a stepping motor are controlled by a PC running the LABVIEW program. The inset shows an example image of a two-dimensional film consisting of a polymer-adsorbed lipid monolayer at the air/water interface with an optically trapped particle attached beneath the film. A more detailed explanation is shown in (c). (b) The chamber used in the experiments. The black lines are a chamber whose components are a dish and a cover, both of which are made of Delrin. Brown double lines represent the heaters. One is for the cover and the other is for the surrounding area. (c) An example of a two-dimensional film at the air/water interface with an optically trapped particle. The film consists of a charged lipid monolayer, a charged polyelectrolyte with the opposite charge, and an optically trapped probe particle bound to the underside of the film. The particle was positioned after the creation of the film at the air/water interface. The distance between the particle and the focal point of an OT, d z , is measured with QPD2.
Distance from the focal point of the OT to the particle center d z vs. the total voltage on a QPD, VZ. The VZ value of QPD2 (IR) is more sensitive to the distance z compared to that of QPD1 (green). The distance d z or the distance from the interface is not quantified. The origin of the z-axis, , is defined as the position which makes the total voltage VZ, the average of the minimum voltage VMIN, and the maximum voltage VMAX. In a linear range between and , the sensitivity using the IR laser is while that with the green laser is
The phase delay and the amplitude are measured at different positions on thez-axis at an oscillation frequency of f = 10 Hz. When a particle is in the range of −5 , the oscillating amplitude of the particle strongly depends on the z-position with no phase delay.
The motion of a particle trapped by oscillating OT is measured at three different oscillating amplitudes. The oscillating voltage sources for a lock-in amplifier are 80 mV, 160 mV, and 240 mV from the lock-in source. Square (black): 80 mV, circle (red): 160 mV, triangle (blue): 240 mV. (a) and (b) Amplitudes and phase delays according to the frequencies at three oscillation amplitudes. (c) The oscillation amplitude of a particle increases linearly according to the bias voltage of a lock-in amplifier with a nearly constant phase delay. The measurements with the feedback system run for about 2.5 h. This shows that the distance d z is nearly constant for at least 2.5 h with a feedback system.
The ratio of the diffusion constant D/D 0. The ratio of diffusion constants of polystyrene particles at the air/water interface were measured using an optical tweezer. The ratio of the drag coefficient to the spring constant of the OT, , was measured via an active method. From the Stoke-Einstein relation, D/D 0 was calculated with the assumption that the spring constant of the OT is constant with a small change in the contact angle.
Confirmation of the optical tweezer if the motion of the optically trapped particle shows the physical properties of the composite – polyelectrolyte/lipid monolayer. (a) The complex drag coefficient of the film shows a significant increase of two decades after adsorbing PSS. Compared to a DODAC monolayer without PSS, the drag coefficient of the DODAC/PSS complex is related to the oscillation frequency ω, and it has an elastic property. (b) The interface shear modulus. At a high frequency, the interface shear modulus is proportional to f 0.66 and the ratio of to is .
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