This paper reviews the latest developments in the design and fabrication of concentration gradient generators for microfluidics-based biological applications. New gradient generator designs and their underlying mass transport principles are discussed. The review provides a blueprint for design considerations of concentration gradients in different applications, specifically biological studies. The paper discusses the basic phenomena associated with microfluidic gradient generation and the different gradient generation modes used in static and dynamic biological assays. Finally, the paper summarizes all factors to consider when using concentration gradient generators and puts forward perspectives on the future development of these devices.

We report a hydroelasticity-based microfluidic oscillator that converts otherwise steady laminar flow to oscillatory flow. It incorporates an elastic diaphragm to enhance nonlinearity of the flow. Negative differential flow resistance is observed. High-frequency oscillatory flow is produced passively through interactions among hydrodynamic, elastic and inertial forces, without resorting to external actuators and control equipment. Driven by fluid flow and pressure, this device can operate in either steady laminar flow or oscillatory flow states, or work as a valve. Its applications for flow control and operation, and mixing enhancement are demonstrated.

In this study, tensile tests of 63Sn/37Pb solder were carried out at various strain rates from 10−5 s−1 to 10−1 s−1 over a wide temperature range from −40°C to 125°C to study the effect of strain rate and testing temperature on the mechanical properties in a systematic manner. Based on these experimental data, a set of empirical formulae was derived by a statistical method to describe the effect of temperature and strain rate in a quantitative manner and explain the variation in the mechanical properties published in other reports. It is concluded that the empirical formulae can be used to characterize the mechanical properties of 63Sn/37Pb over a wide range of temperatures and strain rates.

Paper-based microfluidic devices hold great potential in today's microfluidic applications. They offer low costs, simple and quick fabrication processes, ease of uses, etc. In this work, several wax and paper materials are investigated for the fabrication of paper-based microfluidic devices. A novel method of using wax as a suitable backing to a paper-based analytical device has been demonstrated. Governing equations for the mechanics of the fluid flow in paper-based channels with constant widths have been experimentally validated. Experimental results showing deviations from the governing equations have been verified using fluidic channels with varying widths. There lies the possibility of manipulation of the fluid flow in paper-based microfluidic devices solely using geometric factors. This opens up many potential applications that may require sequential delivery of reagents or samples. Lastly, properties of paper such as the average pore diameter and permeability can be deduced from experimental results.

In this study, a large number of creep tests were carried out to study the effect of stress level and testing temperature on the creep behavior of 63 Sn/37Pb solder in a systematic manner. Based on the dislocation controlled creep mechanism and Gibbs’ free-energy theory, a new creep constitutive model was proposed. The model was found to describe accurately the creep flow of the solder and to be capable of explaining the issues of stress and temperature dependent stress exponent and activation energy in the Arrhenius power-law creep model. Furthermore, the model was employed to predict accurately the long-term reliability of solder joints in a PBGA assembly.