An experimental study of drop-on-demand drop formation

Willis

Review of Scientific Instruments

2015

In this paper, a novel mechanism allowing greater control over the formation of droplets is presented. This is achieved via the use of a dynamic nozzle of adjustable diameter. It is demonstrated that, by using such a nozzle, it is possible to greatly modify the formation and breakup of the ligament behind the main drop, leading to an overall reduction in the number of satellite droplets. Furthermore, by adjusting the delay between the beginning of the forming of the drop and the start of the nozzle constriction, a greater control over both the number of satellites and the size of the main drop can be achieved. It is also shown that only a minimal reduction of the nozzle’s effective diameter is required in order to exploit the positive effects of the technique presented here. This opens the possibility of incorporating the technique into current droplet generator systems, e.g., via the use of piezoelectric driven nozzles or other micro-mechanical actuation technology.

Section: Discussionmentioning

confidence: 99%

Dynamic nozzles for drop generators

Willis

Review of Scientific Instruments

2015

Forensic Science International

“…Therefore, dripping drops with a wide range of volumes can be generated by changing, for example, the diameter or material of a pipette, as already mentioned in the French version of the early BPA work of Balthazard et al [42]. The dripping phenomenon is well understood [61,62], in terms of kinematics [63], frequency, drop volume and -pinch-off length‖, which is the distance where a drop separates itself from its source. Viscosity influences the dripping process of Newtonian [64] and non-Newtonian fluids [65].…”

Section: Fluid Dynamics Descriptionmentioning

confidence: 99%

Fluid dynamics topics in bloodstain pattern analysis: Comparative review and research opportunities

Attinger

Moore

Donaldson

et al. 2013

157

139

This comparative review highlights the relationships between the disciplines of bloodstain pattern analysis (BPA) in forensics and that of fluid dynamics (FD) in the physical sciences. In both the BPA and FD communities, scientists study the motion and phase change of a liquid in contact with air, or with other liquids or solids. Five aspects of BPA related to FD are discussed: the physical forces driving the motion of blood as a fluid; the generation of the drops; their flight in the air; their impact on solid or liquid surfaces; and the production of stains. For each of these topics, the relevant literature from the BPA community and from the FD community is reviewed. Comments are provided on opportunities for joint BPA and FD research, and on the development of novel FD-based tools and methods for BPA. Also, the use of dimensionless numbers is proposed to inform BPA analyses. Keywordsbloodstain pattern analysis, review, dimensionless number, drop generation, trajectory, impact, stain Disciplines Laboratory and Basic Science Research | Other Mechanical EngineeringComments NOTICE: this is the author's version of a work that was accepted for publication in Forensic Science International. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Forensic Science International, [231, 1-3, (2013) Abstract: This comparative review highlights the relationships between the disciplines of bloodstain pattern analysis (BPA) in forensics and that of fluid dynamics (FD) in the physical sciences. In both the BPA and FD communities, scientists study the motion and phase change of a liquid in contact with air, or with other liquids or solids. Five aspects of BPA related to FD are discussed: the physical forces driving the motion of blood as a fluid; the generation of the drops; their flight in the air; their impact on solid or liquid surfaces; and the production of stains. For each of these topics, the relevant literature from the BPA community and from the FD community is reviewed. Comments are provided on opportunities for joint BPA and FD research, and on the development of novel FD-based tools and methods for BPA. Also, the use of dimensionless numbers is proposed to inform BPA analyses.

“…Note that the idea of producing drops on demand is well established in the widely studied context of inkjet printing, see e.g. [10][11][12]. There a pressure pulse, generated for example by a piezoelectric transducer, causes the ejection of a droplet from a nozzle.…”

Section: Drop Formation From a Capillarymentioning

confidence: 99%

Drop Pinch-Off for Discrete Flows from a Capillary

Bierbrauer¹,

Kapur²,

Wilson³

2013

ESAIM: Proc.

Abstract. The problem of drop formation and pinch-off from a capillary tube under the influence of gravity has been extensively studied when the internal capillary pressure gradient is constant. This ensures a continuous time independent flow field inside the capillary tube typically of the Poiseuille flow type. Characteristic drop ejection behaviour includes: periodic drop ejection, drop ejection with associated satellite production, complex dripping, chaotic behaviour and jetting. It is well known that this characteristic behaviour is governed by the Weber (We) and Ohnesorge (Oh) numbers (for a given Bond number) and may be delineated in a We verses Oh operability diagram. An in-depth physical understanding of drop ejection is also of great importance to industry where the tight control of drop size and ejection velocity are of critical importance in industrial processes such as sealants used in electronics assembly and inkjet printing. However, the use of such a continuous flow approach for drop ejection in industry is often impractical since such flows cannot be operator controlled. For this reason it is important to investigate so-called discrete pipe flows where the flow can be turned on and off at will. This means the flow inside the pipe is now time-dependent being controlled in a step-wise fashion. As a first stage in the investigation of drop pinch-off behaviour in discrete pipe flows this paper will study the critical pinch-off time required for drop ejection starting from a pendant drop. This is the discrete amount of time the pipe flow is turned on for in order for a drop to be ejected from the capillary. A Newtonian incompressible free-surface CFD flow code developed at the University of Leeds is used to investigate the critical pinch-off time for a range of internal pipe velocities (the central flow maximum in Poiseuille flow). It is found that the time required for drop ejection to occur decreases exponentially with internal pipe velocity. These characteristic times are also far smaller than typical static drop release times expected from Harkins and Brown analyses. The phenomenology of the process is due to the creation of a capillary wave at the pipe exit upon the sudden turning on of the flow inside the pipe. The capillary wave acts to transport fluid from the upper part of the forming pendant drop at the end of the capillary to the lower part of the drop both lowering the pendant drop centre-of-mass and thinning the neck region connecting the drop to the pipe. This allows the drop to be pinched off at an earlier than expected time as compared to static drop release times.