Using single-beam optical tweezers for the passive microrheology of complex fluids

Abstract

One important aspect of the complete physical characterization of novel viscoelastic materials is the assessment of their response on short timescales. Optical tweezers, equipped with a fast quadrant photodiode, aid in fulfilling this task by providing high-frequency viscoelastic information about the sample. In passive microrheology, this is normally achieved by extracting rheological information from the thermal motion of an optically trapped bead embedded in a test fluid. Here we present the calibration and use of optical tweezers to study the formation of thermally reversible DNA hydrogels. We complement our results with rheological data from dynamic light scattering, video microscopy and conventional bulk rheology. Merging experimental data from different techniques allows us to study the viscoelastic behavior of these DNA networks over a wide frequency-band and the scaling of the complex viscoelastic modulus at the two frequency extremes. By analyzing the high-frequency behavior of our transient network, we prove the semi-flexible polymer nature of DNA and provide an estimate of its persistence length.

On the role of flexibility in linker-mediated DNA hydrogels

“…In agreement with our previous measurements on hydrogels made only of Y-shapes with the appropriate sticky overhangs, 29,30 we observe a liquid-to-gel transition in the Y-L0 system signalling that most sticky ends are hybridised below Tm,YL  52°C, as demonstrated in Figure 2a.…”

“…The plateau in G'(ω) at high frequencies can be used as an estimate of the average mesh size of the gel at that particular temperature (~44 nm), again in agreement with earlier work on Y-shapes only. 29 On further cooling, this mesh size is expected to decrease slightly as the crosslinking density increases, until full hybridisation has taken place. Cooling the sample even further would therefore not deliver higher stiffness as all possible bonds that could form have been saturated.…”

Highly sensitive force measurements in an optically generated, harmonic hydrodynamic trap

“…Using this approach, we obtained apparent trap stiffnesses of 35 ± 3 fN/µ m along the x-axis and 41 ± 4 fN/µ m along the y-axis. These values provide sensitivity that compares favourably to previously reported optical-tweezer estimates [7,9,10]. Thus, optically induced thermoviscous flows appear able to generate highly sensitive traps without the need of any direct laser exposure.…”

“…To obtain a first estimate for the strength of the particle confinement, we analysed the power spectral density (PSD) using a procedure for processing discrete positional data outlined elsewhere [7,8]. The transition between short and long time diffusion was marked in the PSD plot (Fig.…”

“…We used [19] on Fiji. Subsequent analysis involving the calculation of correlation functions was performed on custom-written Matlab routines [7]. Inhouse LabVIEW software combined with custom-written Python routines was used in the trapping experiments to define the scan paths and scan frequency of the infrared laser.…”