Dynamics of semiflexible polymer solutions in the tightly entangled concentration regime

I corroborate an important experimental evidence reported by Schuldt et al. [ Phys. Rev. Lett. 2016, 117, 197801] revealing the incapability of the current theoretical framework to fully describe the dynamics of semiflexible polymer solutions in the tightly entangled concentration regime. These results have been endorsed here by means of previously published, but overlooked, data by Tassieri et al. [ Phys. Rev. Lett. 2008, 101, 198301; Biophys. J. 2008, 94, 2170]. The ensemble of information provides a strong evidence that the scaling law of the plateau modulus as a function of polymer’s concentration and persistence length, i.e., G0 ∝ cαLpβ, should have both the exponents positive, in contrast with all the existing predictions converging on similar values of α > 0 and β < 0—hence the need of new theoretical models able to better interpret the contribution of the polymer bending rigidity to the viscoelastic properties of the polymer network

Linear microrheology with optical tweezers of living cells 'is not an option'!

Optical tweezers have been successfully adopted as exceptionally sensitive transducers for microrheology studies of complex fluids. Despite the general trend, in this article I explain why a similar approach should not be adopted for microrheology studies of living cells. This conclusion is reached on the basis of statistical mechanics principles that indicate the unsuitability of optical tweezers for such purpose

Microrheology with Optical Tweezers of gel-like materials 'is not an option'!

Optical tweezers have been successfully adopted as exceptionally sensitive transducers for microrheology studies of complex 'fluids'. Despite the general trend, a similar approach cannot be adopted for microrheology studies of 'gel-like' materials, e.g. living cells.Comment: 3 pages, 1 figur

Rheology at the micro-scale: new tools for bio-analysis

We present a simple and non-invasive experimental procedure to measure the linear viscoelastic properties of cells by passive particle tracking microrheology. In order to do this, a generalised Langevin equation is adopted to relate the timedependent thermal fluctuations of a probe sensor, immobilised to the cell’s membrane, to the frequency-dependent viscoelastic moduli of the cell. The method has been validated by measuring the linear viscoelastic response of a soft solid and then applied to cell physiology studies. It is shown that the viscoelastic moduli are related to the cell’s cytoskeletal structure, which in this work is modulated either by inhibiting the actin/myosin-II interactions by means of blebbistatin or by varying the solution osmolarity from iso- to hypo-osmotic conditions. The insights gained from this form of rheological analysis promises to be a valuable addition to physiological studies; e.g. cell physiology during pathology and pharmacological response

Dynamic stereo microscopy for studying particle sedimentation

We demonstrate a new method for measuring the sedimentation of a single colloidal bead by using a combination of optical tweezers and a stereo microscope based on a spatial light modulator. We use optical tweezers to raise a micron-sized silica bead to a fixed height and then release it to observe its 3D motion while it sediments under gravity. This experimental procedure provides two independent measurements of bead diameter and a measure of Faxén’s correction, where the motion changes due to presence of the boundary

Frequency dependence of microflows upon acoustic interactions with fluids

Rayleigh surface acoustic waves (SAWs), generated on piezoelectric substrates, can interact with liquids to generate fast streaming flows. Although studied extensively, mainly phenomenologically, the effect of the SAW frequency on streaming in fluids in constrained volumes is not fully understood, resulting in sub-optimal correlations between models and experimental observations. Using microfluidic structures to reproducibly define the fluid volume, we use recent advances modeling the body force generated by SAWs to develop a deeper understanding of the effect of acoustic frequency on the magnitude of streaming flows. We implement this as a new predictive tool using a finite element model of fluid motion to establish optimized conditions for streaming. The model is corroborated experimentally over a range of different acoustic excitation frequencies enabling us to validate a design tool, linking microfluidic channel dimensions with frequencies and streaming efficiencies. We show that in typical microfluidic chambers, the length and height of the chamber are critical in determining the optimum frequency, with smaller geometries requiring higher frequencies

Optical shield: measuring viscosity of turbid fluids using optical tweezers

The viscosity of a fluid can be measured by tracking the motion of a suspended micron-sized particle trapped by optical tweezers. However, when the particle density is high, additional particles entering the trap compromise the tracking procedure and degrade the accuracy of the measurement. In this work we introduce an additional Laguerre–Gaussian, i.e. annular, beam surrounding the trap, acting as an optical shield to exclude contaminating particles

Confinement of surface waves at the air-water interface to control aerosol size and dispersity

The precise control over the size and dispersity of droplets, produced within aerosols, is of great interest across many manufacturing, food, cosmetic, and medical industries. Amongst these applications, the delivery of new classes of high value drugs to the lungs has recently attracted significant attention from pharmaceutical companies. This is commonly achieved through the mechanical excitation of surface waves at the air liquid interface of a parent liquid volume. Previous studies have established a correlation between the wavelength on the surface of liquid and the final aerosol size. In this work, we show that the droplet size distribution of aerosols can be controlled by constraining the liquid inside micron-sized cavities and coupling surface acoustic waves into different volumes of liquid inside micro-grids. In particular, we show that by reducing the characteristic physical confinement size (i.e., either the initial liquid volume or the cavities’ diameters), higher harmonics of capillary waves are revealed with a consequent reduction of both aerosol mean size and dispersity. In doing so, we provide a new method for the generation and fine control of aerosols’ sizes distribution

Optical tweezers: wideband microrheology

Microrheology is a branch of rheology having the same principles as conventional bulk rheology, but working on micron length scales and micro-litre volumes. Optical tweezers have been successfully used with Newtonian fluids for rheological purposes such as determining fluid viscosity. Conversely, when optical tweezers are used to measure the viscoelastic properties of complex fluids the results are either limited to the material's high-frequency response, discarding important information related to the low-frequency behavior, or they are supplemented by low-frequency measurements performed with different techniques, often without presenting an overlapping region of clear agreement between the sets of results. We present a simple experimental procedure to perform microrheological measurements over the widest frequency range possible with optical tweezers. A generalised Langevin equation is used to relate the frequency-dependent moduli of the complex fluid to the time-dependent trajectory of a probe particle as it flips between two optical traps that alternately switch on and off.Comment: 13 pages, 6 figures, submitted to Special Issue of the Journal of Optic

Direct conversion of rheological compliance measurements into storage and loss moduli

We remove the need for Laplace/inverse-Laplace transformations of experimental data, by presenting a direct and straightforward mathematical procedure for obtaining frequency-dependent storage and loss moduli ($G'(\omega)$ and $G"(\omega)$ respectively), from time-dependent experimental measurements. The procedure is applicable to ordinary rheological creep (stress-step) measurements, as well as all microrheological techniques, whether they access a Brownian mean-square displacement, or a forced compliance. Data can be substituted directly into our simple formula, thus eliminating traditional fitting and smoothing procedures that disguise relevant experimental noise.Comment: 4 page