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Zinc oxide (ZnO) has received lot of attention as a nanostructured material because of unique properties rendering it for various applications. Severe drilling problems like pipe sticking, formation damage and high torque and drag are common in extended reach hydrocarbon wells that limit the performance of drilling operations. Bentonite (MMT) is commonly used as a rheology modifier, and attapulgite (ATP) has good Fann viscosity for use in drilling fluids, but both clays face the limitations of flocculation and instability at high temperature and pressure conditions (HTHP). ZnO–clay nanocomposites were formed by incorporating nano‐ZnO in nano‐modified MMT and ATP to maintain remarkable colloidal dispersion and superior rheology, thus enabling homogeneous drilling fluid recipes. ZnO–clay nanocomposites were characterized using scanning electron microscopy and X‐ray diffraction. After successive laboratory investigations, drilling fluid compositions of certain proportions of water and MMT in the presence of ZnO–clay nanocomposite yielded perfect stability at high temperatures and pressure, i.e. stable drilling fluid rheology at HTHP conditions. The best‐recorded results are reported in this paper, and the properties focused here are the plastic viscosity and yield point. Copyright © 2014 John Wiley & Sons, Ltd.

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This paper presents the design procedure of a biomechanical leg, with a passive toe joint, which is capable of mimicking the human walking. This leg has to provide the major features of human gait in the motion trajectories of the hip, knee, ankle, and toe joints. Focus was given to the approach of designing the passive toe joint of the biomechanical leg in its role and effectiveness in performing human like motion. This study was inspired by experimental and theoretical studies in the fields of biomechanics and robotics. Very light materials were mainly used in the design process. Aluminum and carbon fiber parts were selected to design the proposed structure of this biomechanical leg, which is to be manufactured in the Mechanical Lab of the Sultan Qaboos University (SQU). The capabilities of the designed leg to perform the normal human walking are presented. This study provides a noteworthy and unique design for the passive toe joint, represented by a mass-spring damper system, using torsion springs in the foot segment. The working principle and characteristics of the passive toe joint are discussed. Four-designed cases, with different design parameters, for the passives toe joint system are presented to address the significant role that the passive toe joint plays in human-like motion. The dynamic motion that is used to conduct this comparison was the first stage of the stance motion. The advantages of the presence of the passive toe joint in gait, and its effect on reducing the energy consumption by the other actuated joints are presented and a comparison between the four-designed cases is discussed.