We provide a multiple-sinusoid modulated optical tweezers based method to perform microrheology of a sample over a large frequency band in a one shot. So we feed the trapped particle with a square wave and a superposition of multiple-sinusoid of specific frequency and amplitude to achieve our purpose towards active microrheology. To maintain the SNR upto a certain level for the entire frequency range we increase the amplitude at high frequency in our setup. We measure the parameters of the complex fluid by extracting the phase responses at each frequency of the particle with a high SNR value compared to passive microrheology. We perform this method for a various concentrations of polyacrylamide-water solution and obtain a good agreement of the fluid parameters with the theoretical values.
Surface effects are crucial in several mesoscopic phenomena, especially those concerning biological entities. Here we determine the effects of Van der Waals forces at relatively long range ( 80 nm) by optically trapping a probe particle close to a large silica particle and modulating the spatial position of the probe employing oscillating optical tweezers. This method has greater signal-to-noise in the experimentally measured probe-response as compare to that obtained from measurements of Brownian fluctuations. We quantify the H-value experimentally by analyzing the amplitude response of a single trapped particle in comparison to numerically expected results by employing chi-square fitting, and obtain good agreement with the known H-value for the system.
Two-point microrheology measurement from widely separated colloids can reveal the bulk rheological property of a fluid. We develop such a technique which measures the effective viscosity using two trapped particles in a dual trap optical tweezers by exploiting the motional resonance excited in the probe particle when the control particle is driven externally. We carry out the measurement both from the amplitude and the phase response of the resonance and show that the zero-crossing of the phase with respect to the drive signal at the resonance frequency gives more accurate results when the particles are separated widely. Later on, we compare our measured viscosity values with that measured using a commercial rheometer and obtain an agreement within ∼1 %. In future, this method can be extended to a linear viscoelastic fluid enabling high accuracy measurements.
We have developed a new technique of optical micro-patterning using micro bubbles based on light induced self-
assembly of materials using thermo-optic tweezers. Presently, we use a liquid matter- Soft Oxo-Metalate (SOMs)
that have high absorption near the wavelength of the tweezers laser at 1064 nm. An aqueous dispersion of the
sample solution is taken in a glass sample chamber and introduced into the translation stage of our tweezers
set-up (inverted microscope) where a highly focused laser beam is aimed at SOM particles adsorbed on the top
surface of the sample chamber. The high absorptivity of SOMs ensures the creation of a local `hot-spot' which
leads to the nucleation of a micro-bubble in this region. Thus, a large local surface-tension gradient is introduced
in the vicinity of the micro-bubble due to the temperature gradient produced at the two ends of the bubble, which
leads to a Marangoni type convective
ow around the bubble. This
ow causes material to be self-assembled
at the base of the bubble. As the translation stage is moved, the `hot-spot' moves simultaneously, and due
to the resulting
ow dynamics, the microbubble is also translated thus causing continuous accumulation of the
SOMs around it. Simultaneously, due to the sudden thermal shock generated when the `hot-spot' is moved away
from the self-assembled SOMs, they undergo a phase transition from soft (liquid) to hard (crystalline) state,
resulting in the formation of a stable permanent pattern of choice on the glass substrate. This technique can
have diverse applications with materials other than SOMs including carbon nano tubes, organic dyes, catalysts
and conducting polymers, etc, being co-deposited from aqueous dispersions of the particular material with the
SOMs. The patterns thus formed have been used for various applications including the development of catalytic
micro-chips, and solution processed printable micro-circuits.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.