Turner Research Group

 
 

 

Research Overview

The Turner Research Group investigates problems at the nexus of mechanics, manufacturing, and materials, with a particular emphasis on research questions involving small-scale systems and interfaces. In general, the research projects in our group involve a combination of analytical modeling, computational simulations, and experimental measurements. Some active areas of research are:

  • Surfaces and materials with tunable mechanical properties, including variable adhesion, friction, and stiffness
  • Nanocomposite coatings and heterogenous materials with high fracture resistance
  • Mechanics of microtransfer printing, wafer bonding, and advanced patterning processes for semiconductors
  • Design and manufacturing of flexible hybrid electronics and sensors
  • Failure and reliability of micro- and nano-systems, including AFM probes and flexible electronics
  • Microscale systems for probing the mechanics of biological interfaces and cells
  • Experimental, theoretical, and computational contact and adhesion mechanics

Research Highlights

Please see our Publications for additional detail on our research. Work from a few publications is highlighted below:

Printing and mechanical characterization of cellulose nanofibril materials
L.M. Mariani, W.R. Johnson, J.M. Considine and K.T. Turner, Cellulose, 26, 2639-2651 (2019). http://doi.org/10.1007/s10570-019-02247-w

Printing of nanocellulose

Cellulose nanofibrils (CNF) are a promising building block of structural materials because they are biodegradable, can be made into optically transparent bulk materials, and have exceptional specific strength and stiffness compared to common synthetic polymers. The manufacturing of bulk materials from CNFs is a challenge because CNFs form networks in solution at low solids concentration, which can result in long processing times as well as large residual stresses and distortion upon water removal. Here, a method to form materials from CNF suspensions via direct ink writing, a type of additive manufacturing, is demonstrated. Multilayer printing of CNFs provides a route to control drying time by depositing thin layers one at a time. A printing system with a pressure-controlled dispensing system was used to deposit aqueous CNF suspensions onto a temperature-controlled substrate. The geometry, roughness, and mechanical properties of the printed structures were characterized. The shape of the printed line profile is controlled by a combination of the wettability of the substrate, dispense rate, printing speed, and temperature of the substrate. Spatial variation of the elastic modulus of printed CNF structures was assessed with nanoindentation and the average percent difference was found to be small at 2.6% of the mean over the area of the printed lines. Through multilayer printing freestanding films with thicknesses greater than 60m were achieved. Tensile specimens were printed and characterized; a tensile strength of 72.6MPa +/- 7.4MPa and a Young's modulus of 10.2GPa +/- 1.2GPa were measured.

 

Toughening nanoparticle films via polymer infiltration and confinement
Y.J. Jiang, J.L. Hor, D. Lee and K.T. Turner, ACS Applied Materials & Interfaces, 10, 44011-44017 (2018). http://doi.org/10.1021/acsami.8b15027

Fracture of micropillars via nanoindentation

Disordered nanoparticle films have significant technological applications as coatings and membranes. Unfortunately, their use to date has been limited by poor mechanical properties, notably low fracture toughness, which often results in brittle failure and cracking. We demonstrate that the fracture toughness of TiO2 nanoparticle films can be increased by nearly an order of magnitude through infiltration of polystyrene into the film. The fracture properties of films with various polymer volume fractions were characterized via nanoindentation pillar-splitting tests. Significant toughening is observed even at low volume fractions of polymer, which allows the nanoparticle packing to be toughened while retaining porosity. Moreover, higher-molecular-weight polymers lead to greater toughening at low polymer volume fractions. The toughness enhancement observed in polymer-infiltrated nanoparticle films may be attributed to multiple factors, including an increase in the area and strength of interparticle contacts, deflection and blunting of cracks during failure, and confinement-induced polymer bridging of nanoparticles. Our findings demonstrate that polymer infiltration is a highly effective route for reinforcing nanoparticle packings while retaining porosity.

 

Composite microposts with high dry adhesion strength
H.K. Minsky and K.T. Turner, ACS Applied Materials & Interfaces, 9, 18322-18327 (2017). http://doi.org/10.1021/acsami.7b01491

Composite posts with enhanced adhesion

Interfaces with enhanced and tunable adhesion have applications in a broad range of fields, including microtransfer printing of semiconductors, grippers on robots, and component handling in manufacturing. Here, a composite post structure with a stiff core and a compliant shell is used to achieve an enhanced adhesion under normal loading. Loading the composite structure in shear significantly reduces the effective adhesion strength, thus providing tunability. The composite posts can be used as stamps in microtransfer printing processes or as building blocks of large-area tunable surfaces composed of arrays of posts. Experimental measurements on composite posts with diameters of 200 micrometers show a peak adhesion strength of 1.5 MPa, a 9 times enhancement in adhesion relative to a homogeneous post under normal loading, and also that the adhesion can be reduced by nearly a factor of 7 through the application of shear. The adhesion behavior of these composite structures was also examined using finite element analysis, which provides an understanding of the mechanics of detachment. Finally, the composite adhesive posts were used as stamps in a microtransfer printing process in which micrometers thick silicon membranes were retrieved and subsequently printed.

 

 

Copyright Kevin T. Turner 2005-2022