New Mechanical Probes

We have developed several new approaches for characterizing the mechanics, dynamics and structure of soft materials:

1. Capillary Micromechanics - Testing the mechanics of microscopic soft objects

We compress soft objects like microgel particles or biological cells in microscopically tapered glass capillaries. By monitoring their deformation under the microscope we obtain information on the mechanics of these microscopic soft objects.

2. Strain-Rate Frequency Superposition - A new approach to oscillatory rheology

We perform oscillatory rheological measurements on soft glassy materials at fixed strain-rate amplitude. Our experiments indicate that the strain rate speeds up the slow structural relaxation process of the glassy materials, moving it towards higher frequencies. Surprisingly, the shape of the relaxation remains unchanged as the applied strain rate is increased. This suggests that such constant-rate measurements can be used to extend the frequency range accessible to oscillatory measurements for soft glassy materials (analogous to time-temperature superposition in molecular and polymeric glasses, where relaxation processes are sped up by increasing the temperature).

3. Compression and re-swelling after an osmotic shock

We use dedicated microfluidic devices to subject soft hydrogel particles to a rapid change in osmotic pressure, an osmotic shock. Doing so, under some conditions we observe a surprising behavior where the particles undergo a reversible compression behavior after being subjected to a jump in osmotic pressure - after an initial rapid compression, we observe a slow re-swelling process, where particles slowly reswell to nearly their initial volume. We attibute this slow process to the diffusion of the osmolyte, the macromolecules used to apply the osmotic pressure, into the microgel particle. Our simple model of the process accounts for the experimental data and enables us to extract 3 important properties from a single osmotic shock measurement: The compressive (bulk) modulus of the particles, their permeability to the solvent, and the diffusion coefficient of the osmolyte molecules within the gel network.

4. Mechanics from Calorimetry

We use calorimetric measurements to probe the mechanics of temperature-sensitive soft particles such as pNIPAM hydrogels and microgels. This mechanical test is unusual in that it does not involve a direct measurement of any forces of stresses in the material. The method enables us to measure the mechanics of sub-micron or oddly shaped particles, for which standard mechanical measurements are difficult to employ.

Capillary Micromechanics

Microscopic Mechanics of Soft Objects

Understanding the macroscopic properties of soft materials often requires knowledge of the local mechanical response of the material. We are especially interested in the study of suspensions of soft objects, where macroscopic rheological measurements indicate significant deviations from the behavior known from hard particle suspensions. One example is the experimental observation of a strongly accentuated shear thinning behavior for suspensions of soft spheres as compared to hard spheres. The basis for an understanding of such behavior is the possibility to experimentally access the mechanical properties of the soft objects at the single particle level.

A typical "squeezing experiment".

A soft microgel particle is deformed in a tapered glass capillary, as the inlet pressure is gradually increased (red markers indicate the particle position for clarity).

Experimental approach

We have developed a new way to test the mechanical properties of single soft objects. Our approach is based on a simple microfluidic device, a tapered glass capillary. In a typical experiment, a dilute suspension of soft particles is pumped into this capillary by applying a small overpressure at the inlet, as shown schematically in Fig. 1. At small enough pressure the capillary will eventually get blocked by a single soft object, as shown in the bottom picture in Fig. 1. In this situation, the entire pressure difference between inlet and outlet is balanced by the stress exerted on this single soft particle.

This allows us to monitor the mechanical response of this particle by monitoring its deformation as a result of the applied pressure difference.

More details on this approach can be found on the "Capillary micromechanics page" [Link to follow soon..].


Ian Robb (Baroid Technologies)
Zhibing Hu (University of North Texas)


Coming soon ..

Strain-Rate Frequency Superposition (SRFS)

A new approach to oscillatory rheology

We have developed a new rheological probe that naturally combines linear and nonlinear rheological experiments in a universal picture. Our new technique allows us to study the structural relaxation of soft glassy materials in oscillatory rheological measurements.

Materials we are interested in

We study soft, dense materials whose rheological behavior is dominated by a structural relaxation process. Examples are:
  • Emulsions
  • Foams
  • Suspensions of Hydrogel Particles
  • Suspensions of Solid Colloidal Particles

Constant-rate frequency sweeps shifted onto a single master curve. The shiftfactors shown in the inset provide information on the shear rate dependence of the average relaxation time of the system.

For more information on this project, see the "SRFS page".


Kunimasa Miyazaki (Kochi University of Technology, Japan)
Johan Mattsson (Chalmers University, Sweden)
David Weitz (Harvard University)
David Reichman (Columbia University)
Zhibing Hu (University of North Texas)