Negative Friction Force Research Articles

Dynamic scanning force microscopy investigation of nanostructured spiral-likedomains in Langmuir–Blodgett monolayers

The advantages/limitations obtained by working in dynamic scanning forcemicroscopy (DSFM) at different levels of tip/sample interaction forces (from thenet attractive to the hard repulsive regime) are experimentally shown by imagingmonolayers containing novel fibre-like supramolecular nanostructures wrapped upin spiral-like domains. The structures have been obtained by using theLangmuir–Blodgett technique and transferring onto mica monolayers ofquercetin-3-O-palmitate molecules mixed with a fraction of about 25% ofdimyristoylphosphatidylcholine. The measurements in the attractive regimeallowed us to reveal morphological features of the supramolecular structures thatcannot be demonstrated by the conventional repulsive regime. In particular, byattractive dynamic scanning force microscopy (DSFMA) the height ofthe fibre-like structures was a factor of two to three higher and peculiarnano-ruptures along the wrapped fibres have been observed. The influence of thetip/sample contact interaction was investigated by recording images in hardtapping and successively imaging the same region in DSFMA as well as bycomparing the images in DSFMA with those obtained in negative lift modeforce modulation microscopy, phase imaging and friction force microscopy.

Piles in Punched Holes with Surrounding Soils Subject to Slump-Type Settlement

Equipment has been developed and tested for the installation of piles in punched holes with a diameter to 1 m in soils classed as Type II in terms of proneness to slump-type settlement. A field experiment was conducted to investigate single and grouped piles subject to prolonged wetting and slump-type settlements of the surrounding soils under their own weight. The effect of compaction of the soil around the pile on the reduction in negative-friction forces and settlement of piles in punched holes is noted.

Experimental verification of the mechanisms causing stick–slip motion originating from relative deceleration

In literature, the stick–slip phenomenon is usually explained from an initial stick-phase. The stability conditions with respect to stick–slip (the occurrence or not) are also derived by checking the reoccurrence of a relative standstill (stick-phase) after the slip-phase that has been following an initial stick-phase. However, stick–slip can also arise from an initial situation of pure slip. In this paper, it is experimentally shown that friction induced tangential oscillations during stick–slip can merge into stick–slip during an impressed deceleration, if their amplitude is sufficiently large. Owing to the interaction between friction phenomena and the dynamic behaviour of the mechanical system of which the friction interface is a part, the (negative) friction force–relative speed gradient (d F/d v rel), which gives rise to the tangential oscillations and has to be considered for the study of stick–slip, must be derived from changes of the relative speed with the same time-constant as the mechanical system. It is also experimentally shown that the risk of stick–slip during deceleration is enhanced by an increase of the time-length of deceleration and by an increase of the tangential stiffness of the mechanical system. The correlation of the experimental results with previous theoretical analysis is clarified. As a result, some widely accepted models for the friction as a function of the relative speed have to be put in question. It is also shown that the major problem in determining the gradient d F/d v rel is the inadequateness of the classical friction test rigs.

Loads in Model Grain Bins as Affected by Filling Methods

Measurements of wall and bottom loads in flat-bottom model bins have shown the influence of filling methods and wall surface on wall loads and the static-to-dynamic load shift. Four spout-filling methods, at center and eccentric locations, sprinkle filling, and uniformly filling through an annular ring near the bin wall, were evaluated. Three types of bin walls were tested for each filling method: smooth galvanized steel, corrugated galvanized steel, and smooth steel covered with abrasive cloth. Soft and hard winter wheats were used in the experiments. The static wall load to total grain load ratio (SWL/TGL) and the dynamic-to-static wall load ratio (DSR) were found to be influenced by the filling method. In general, a higher SWL/TGL resulted in a lower DSR. Sprinkle filling produced lower SWL/TGL values on the smooth wall and higher values on the rough and corrugated walls. The ratio of the wall to total grain load at the start of discharge was highest for the first opening of the discharge gate as compared to two successive gate openings (each after 30 min of rest). The filling method and the type of wheat significantly influenced the negative friction force on the smooth wall bin. Negative friction force values were highest for the top filling methods. A negative friction force was not observed for corrugated and rough wall bins.

Field investigations of negative friction forces in thawing soils : Soil Mech Found Engng, V15, N4, July–Aug 1978, P241–245

Field investigations of negative friction forces in thawing soils : Soil Mech Found Engng, V15, N4, July–Aug 1978, P241–245

Field investigations of negative friction forces in thawing soils

1. The negative friction forces can be estimated by calculation with the use of the Shvedov-Bingham equation of deformation of a viscoplastic body on the basis of the results of pull-out testing of an experimental foundation with preliminary thawing of the frozen ground around this foundation. 2. The negative friction forces exceed the ultimate shear strength of the thawed soil on the skin surface of the foundation and depend on the rate of mutual movement of the thawing soil and foundation. 3. In connection with the decrease in the rate of thawing of permafrost under a structure with the course of time and the increase of the depth of thawing, the effect of negative friction forces should be checked for the most unfavorable combination of these parameters. 4. The minimum values of the negative friction forces obtained in experiments for conditions of the movement of the thawing front parallel to the axis of the pile or support exceed by a factor of 2–3 their value set by the standards [1, 2].

Experimental study of negative skin friction of piles in the case of slumping of soils from their own weight

1. Investigations were carried out to determine for the first time by direct measurement the magnitude of the loading (negative) friction forces of piles in the case of slumping of soils under their own weight. The maximum magnitude of these forces in the given experiment was 282 kN.