Ningyu Liu

Electrical conductivity of thermoresponsive shape-memory polymer with embedded micron sized Ni powder chains

The electrical resistivity of a thermoresponsive polyurethane shape-memory polymer (SMP) filled with micron sized Ni powders is investigated in this letter. We show that, by forming conductive Ni chains under a weak static magnetic field (0.03T), the electrical conductivity of the SMP composite in the chain direction can be improved significantly, which makes it more suitable for Joule heat induced shape recovery. In addition, Ni chains reinforce the SMP significantly but their influence on the glass transition temperature is about the same as that of the randomly distributed Ni powders.

Formation of micro protrusion arrays atop shape memory polymer

Utilizing the significant shape recovery ability (in the order of 100% strain) in shape memory polymers (SMPs), we propose two simple approaches, namely laser heating and indentation, to produce micro-sized protrusion arrays. In the former, after local laser heating a pre-compressed SMP, protrusive bumps can be produced. In the latter, through an indentation-polishing-heating process, various shaped protrusive bumps can be produced. It is to demonstrate that indentation is a more convenient and powerful approach than laser heating, since well controlled, different shaped protrusion arrays can be realized.

A slip model with molecular dynamics

In this paper we present analytical investigations of the slip flow in a hard disk drive where the flying head is approximately 25 nm above the rotating disk. A new slip velocity model is developed, incorporating molecular dynamics to take into account the impact of molecular collisions which play an important role in the interactions between molecules and the solid surface. A modified Reynolds equation is derived based on the modified slip model. Analytical solutions for velocity distribution and flow rate, using the modified Reynolds equation are obtained. Non-dimensional flow rate for plane Poiseuille flow, pressure distributions and load-carrying capacities for slope flow are compared with those available in the literature. Comparison with first-order, second-order and 1.5-order slip models shows that the new model agrees reasonably well with the solution obtained from the linearized Boltzmann equation.

The posture effects of a slider air bearing on its performance with a direct simulation Monte Carlo method

The direct simulation Monte Carlo method is used to study the gas slider bearing problem in a magnetic recording storage system. The flow field of a micro-channel between an inclined slider and a rotating disk is calculated under a variety of slider postures and radial positions relative to the spinning disk. The effects of the windward angle of the flying slider and the lower plate velocity of the channel on two-dimensional pressure distributions and velocity profiles are presented. The relationship of load capacity and the windward angle, and the position of the resultant force of the loading capacity, are also obtained, which can be used to optimize the design of a floating head mechanism that is composed of the slider bearing and its suspension system. The slip velocities and the shear stress are further investigated in order to determine the near-wall feature of micro-channel flow in the transition regime.

The glass transition temperature of polyurethane shape memory polymer reinforced with treated/non-treated attapulgite (playgorskite) clay in dry and?wet conditions

Attapulgite (playgorskite), a kind of nanosized fibrous clay mineral, may provide a simple and cheap alternative to improve the stiffness and actuation stress of shape memory polymers (SMPs). As a first step, in this paper, we investigate the glass transition temperature of a polyurethane SMP reinforced with treated/non-treated attapulgite in wet and dry conditions. In addition to confirming the strong influence of moisture, the results reveal that non-treated clay significantly reduces the glass transition temperature (Tg) of the composites, while the influence of treated clay on Tg is limited. However, for composites mixed with non-treated clay, after drying, the well pre-wetted samples have a much higher Tg than that of the dry ones. A partial detachment mechanism is proposed to explain this interesting phenomenon.

A generic approach for producing various protrusive shapes on different size scales using shape-memory polymer

We demonstrate a generic approach for producing different shaped and sized protrusive features using shape-memory polymer (SMP). Utilizing the large recoverable strain in the SMP, and depending on the pre-straining of the SMP, shape of the indenter, temperature during indenting and the depth of polishing, we show that a variety of features of different sizes can be easily obtained.

The formation of micro-protrusions atop a thermo-responsive shape memory polymer

We present an experimental investigation on a novel approach to produce micro-sized protrusive features atop a thermo-responsive shape memory polymer (SMP). This approach includes three steps, namely, indenting atop an SMP sample (using a Berkovich indenter in this study), polishing and then heating it for full shape recovery. Apart from ordinary samples, some SMP samples are pre-stretched in the in-plane direction or pre-compressed in the out-of-plane direction. The relationships between the height/shape of protrusion and the depth of indent are obtained for all indents in samples with/without pre-straining. Intrinsic relationships among the indentation depth, polishing depth and height/shape of protrusion are further revealed quantitatively in a dimensionless manner. The influence of pre-straining is discussed.

Stress-density ratio slip-corrected Reynolds equation for ultra-thin film gas bearing lubrication

The direct simulation Monte Carlo method is used to evaluate the improved slip boundary condition and slip-corrected Reynolds equation for gas lubrication in magnetic recording storage system. The study shows that in transition regime, the slip velocity is affected not only by shear stress, but also by density itself. In this ultra-thin film lubrication environment, the slip velocity has a linear relationship with the ratio of shear stress to density near the wall. This observation is particularly true when Knudsen number is around 1 or larger. Through this study, the authors have developed a new slip velocity model, named stress-density ratio model, particularly suitable to transition regime. Based on this novel model, a slip-corrected Reynolds equation is also derived and solved numerically. The stress-density ratio model is confirmed to offer a better applicability than the previous slip-flow models for modern computer magnetic storage device, where the Knudsen number of slider air bearing is in the ra...

Formation of Protrusive Micro/Nano Patterns atop Shape Memory Polymers

The surface morphology of materials is of fundamental importance to many applications (e.g., surface wetting, friction, surface roughness, reflection, drag, adhesion, etc). Various approaches for micro/nano patterning atop polymer surfaces have been proposed in recent years. However, a cost effective technique is still highly in demand. In this paper, we demonstrate a few novel but rather simple and generic approaches for surface micro/nano patterning using shape memory polymers (SMPs). Reversible micro vertical chains, crown shaped protrusion arrays and strip/labyrinth wrinkles atop SMPs are presented.

The impacts of time-step size in the application of the direct simulation Monte Carlo method to ultra-thin gas film lubrication

The size of the time step is an important factor in the application of the direct simulation Monte Carlo (DSMC) method for rarefied gas dynamics. In order to understand the effects of this essential factor and then to apply the DSMC method more effectively, the influence of the size of time step used on the accuracy of the algorithm is demonstrated and investigated. It is found that some wall properties related to the streamwise derivative of velocity, such as wall shear stress, are sensitive to the size of time step. A series of computation and analysis is presented to illustrate the computational errors (bias) caused by the size of time step and the corresponding computational resource applied. From this investigation, a compromise can be made between the magnitude of error and computing cost; thus, a criterion on the size of time step is proposed for the DSMC method. This criterion is necessary for the application of the DSMC method used in a low subsonic, ultra-thin gas film lubrication with a characteristic scale of one mean free path, such as the gas bearing flow under the floating slider.