Jeffrey L. Hutter

Calibration of atomic‐force microscope tips

Images and force measurements taken by an atomic‐force microscope (AFM) depend greatly on the properties of the spring and tip used to probe the sample’s surface. In this article, we describe a simple, nondestructive procedure for measuring the force constant, resonant frequency, and quality factor of an AFM cantilever spring and the effective radius of curvature of an AFM tip. Our procedure uses the AFM itself and does not require additional equipment.

The genome of Tetranychus urticae reveals herbivorous pest adaptations

The spider mite Tetranychus urticae is a cosmopolitan agricultural pest with an extensive host plant range and an extreme record of pesticide resistance. Here we present the completely sequenced and annotated spider mite genome, representing the first complete chelicerate genome. At 90 megabases T. urticae has the smallest sequenced arthropod genome. Compared with other arthropods, the spider mite genome shows unique changes in the hormonal environment and organization of the Hox complex, and also reveals evolutionary innovation of silk production. We find strong signatures of polyphagy and detoxification in gene families associated with feeding on different hosts and in new gene families acquired by lateral gene transfer. Deep transcriptome analysis of mites feeding on different plants shows how this pest responds to a changing host environment. The T. urticae genome thus offers new insights into arthropod evolution and plant–herbivore interactions, and provides unique opportunities for developing novel plant protection strategies.

Erratum: ‘‘Calibration of atomic‐force microscope tips’’ [Rev. Sci. Instrum. 64, 1868 (1993)]

In our calibration of atomic-force microscope cantilevers, we neglected to correct for the frequency response of the optical-detection electronics. The response to cantilever vibrations will have a high-frequency cut-off, which, in our case, was higher than the resonant frequency of the cantilever. Our results were not affected, but for higher resonant frequencies, one should calibrate the detector response. We thank V. Croquette for raising this point.

Manipulation of van der Waals forces to improve image resolution in atomic‐force microscopy

Although the atomic force microscope (AFM) resembles superficially the scanning tunneling microscope (STM), its imaging resolution is in general much coarser. For the AFM, long‐range interactions—most notably the van der Waals force—imply that image resolution is set by the macroscopic tip radius rather than by a single atom, as with the STM. Experimentally, we show that van der Waals forces can be measured using an AFM. By immersing tip and sample in an appropriate fluid, we can effectively eliminate the van der Waals force, leading to a marked improvement in AFM image quality.

Polymer conformations of gas-hydrate kinetic inhibitors: A small-angle neutron scattering study

We have used small-angle neutron scattering to characterize the polymer conformations of four nonionic water soluble polymers: poly(ethylene oxide), poly(N-vinyl-2-pyrollidone), poly(N-vinyl-2-caprolactam), and an N-methyl, N-vinylacetamide/N-vinyl-2-caprolactam copolymer. The last three of these are able to kinetically suppress hydrate crystallization, and their inhibitor activity ranges from moderate to very effective. This attribute is of significant commercial importance to the oil and gas industry, but the mechanism of the activity is unknown. The dilute-solution polymer conformation in a hydrate-forming tetrahydrofuran/water fluid shows little difference among the four polymers: the majority of the scattering is that expected for a polymer in a good solvent. Each solution also exhibits some additional low-q scattering which we attribute to aggregates. In the presence of a hydrate-crystal/liquid slurry, the three inhibitor polymers significantly change their conformation. Utilizing results from our p...

Measurement and manipulation of van der Waals forces in atomic‐force microscopy

An understanding of the interaction between tip and sample in atomic‐force microscopy is needed to interpret atomic‐force‐microscope (AFM) images. In contact mode, the strength of the van der Waals (vdW) force sets image resolution; in noncontact mode, local gradients in the vdW force are imaged. By immersing tip and sample in an appropriate fluid, we can decrease the vdW forces and even change their sign. Selecting a fluid that leads to a small repulsive vdW force can greatly improve image resolution and eliminates problems caused by the well‐known tip‐snapping instability. To measure the vdW interactions produced by different fluids, we have developed ways to calibrate the spring constant and sharpness of AFM tips and to measure accurately the Hamaker constant of vdW interactions. These techniques show that the AFM can be used for local force measurements with an accuracy approaching that of surface‐force apparatuses. As an example, we have observed the crossover from nonretarded to retarded vdW forces ...

Banded spherulitic growth in a liquid crystal

Abstract Spherulitic growth produces radial arrays of polycrystalline aggregates in a wide variety of materials, ranging from pure elements to macromolecules. Despite more than a century of study, a generally accepted theory of spherulitic growth is lacking; indeed, the existence of a common mechanism is debated. One commonly seen subset of this growth morphology — banded spherulites — exhibits bands concentric about the spherulite centers. These bands have been well characterized in polymer spherulites, but remain poorly understood. Comparisons of such bands in a diverse variety of materials will help establish a general mechanism for their formation. Here we present a detailed optical and atomic-force microscopy study of banding in a liquid crystalline material.

Control of surface topography in biomimetic calcium phosphate coatings.

The behavior of cells responsible for bone formation, osseointegration, and bone bonding in vivo are governed by both the surface chemistry and topography of scaffold matrices. Bone-like apatite coatings represent a promising method to improve the osteoconductivity and bonding of synthetic scaffold materials to mineralized tissues for regenerative procedures in orthopedics and dentistry. Polycaprolactone (PCL) films were coated with calcium phosphates (CaP) by incubation in simulated body fluid (SBF). We investigated the effect of SBF ion concentration and soaking time on the surface properties of the resulting apatite coatings. CaP coatings were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), and energy dispersive X-ray spectrometry (EDX). Youngs modulus (E(s)) was determined by nanoindentation, and surface roughness was assessed by atomic force microscopy (AFM) and mechanical stylus profilometry. CaP such as carbonate-substituted apatite were deposited onto PCL films. SEM and AFM images of the apatite coatings revealed an increase in topographical complexity and surface roughness with increasing ion concentration of SBF solutions. Youngs moduli (E(s)) of various CaP coatings were not significantly different, regardless of the CaP phase or surface roughness. Thus, SBF with high ion concentrations may be used to coat synthetic polymers with CaP layers of different surface topography and roughness to improve the osteoconductivity and bone-bonding ability of the scaffold.

Simultaneous measurement of Young’s and shear moduli of multiwalled carbon nanotubes using atomic force microscopy

Carbon nanotubes (CNTs) are widely hailed as the strongest material known to mankind. However, experimental measurements—and even theoretical estimates—of their mechanical properties span a wide range. We present an atomic force microscopy study of multiwalled CNTs, which, unlike previous such studies, measures the tube compliance as a function of position along suspended tubes. This permits a simultaneous determination of the effective Young’s and shear moduli of CNTs: 350±110 and 1.4±0.3GPa, respectively.

Frequency-dependent viscoelasticity measurement by atomic force microscopy

We demonstrate a new technique for investigating viscoelastic properties of soft materials using the atomic force microscope. A small oscillatory voltage is added to the deflection signal of the atomic force microscope causing a vertical oscillatory sample motion. Monitoring the amplitude and phase of this motion allows determination of the viscous and elastic moduli of the sample as a function of frequency during contact imaging. This technique is applied to suspended poly(vinyl alcohol) nanofibers and poly(vinyl alcohol) hydrogels, giving results similar to those measured using traditional static methods. However, the moduli of both the fibers and the hydrogels show a significant frequency dependence. The Youngs modulus of the fibers increases with frequency, while for the viscoelastic hydrogels, the storage modulus dominates the mechanical response at low frequency whereas the loss modulus dominates at high frequency.