Touching the Micron: Tactile Interactions with an Optical Tweezer Stuart Lamont 1 stuartalamont@gmail.com John Williamson 1 jhw@dcs.gla.ac.uk Richard Bowman 2 r.bowman@physics.gla.ac.uk Roderick Murray-Smith 1 rod@dcs.gla.ac.uk Matthias Rath 1 matthias.rath@tu-berlin.de Miles Padgett 2 Miles.Padgett@glasgow.ac.uk 1. School of Computing Science, 2. School of Physics, University of Glasgow, G12 8QQ, Scotland. ABSTRACT A tablet interface for manipulating microscopic particles is augmented with vibrotactile and audio feedback.The feed- back is generated using a novel real-time synthesis library based on approximations to physical processes, and is ef- ficient enough to run on mobile devices, despite their lim- ited computational power. The feedback design and usabil- ity testing was done with a realistic simulator on appropri- ate tasks, allowing users to control objects more rapidly, with fewer errors and applying more consistent forces. The feed- back makes the interaction more tangible, giving the user more awareness of changes in the characteristics of the op- tical tweezers as the number of optical traps changes. ACM Classification Keywords H.5.2 User Interfaces: Input devices and strategies Author Keywords Tactile feedback; optical tweezers; real-time synthesis. INTRODUCTION We apply developments in mobile multimodal feedback de- sign [3, 12] to augment the tablet controller of an Opti- cal Tweezer, described in [2], by augmenting the visual and touch-based interaction with vibrotactile and audio feedback. These feedback the level of force control users have over the microscopic objects being manipulated, allowing them to bet- ter control their finger movements. We also introduce an op- tical tweezer simulation for training and interaction design. Optical Tweezers Many areas of science and engineering are increasingly con- cerned with micro- and nanometre length scales. The abil- ity to manipulate microscopic objects is, therefore, an im- portant tool in fields ranging from biology to nanofabrica- tion. This can be achieved by passing a tightly focussed laser beam through a small transparent object, where any deflec- tion of the light results in a change in the momentum carried by the beam. This change in momentum results in a force which, for most objects, acts to pull the object towards the Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. MobileHCI’12, September 21-24, 2012, San Francisco, CA, USA. Copyright 2012 ACM 978-1-4503-1105-2/12/09...$10.00. Figure 1: a) Running an experiment with the simulator on a tablet, with a user manipulating particles. b) Plots indicating user behaviour in one of the experiments. focal point of the beam (hence the name ’trap’), and was first used to manipulate micron-sized objects by Ashkin [1]. This technique has been extended and refined to make precise force and displacement measurements [7], providing impor- tant insights into biophysical problems such as muscle mo- tors and DNA dynamics [7]. Holographic Optical Tweezers (HOT) [9], provide an extension of this technique which uses diffractive optics to provide interactive 3D control over mul- tiple particles [5]. Interaction challenges Guiding several particles dynamically, simultaneously along trajectories is a non-trivial problem, not easily solved with a mouse or joystick based interface. Multi-touch tablets or surfaces allow the user to simultaneously manipulate multi- ple particles, overcoming a limitation of mouse-based inter- faces. The tablet form factor used here is more convenient and affordable than the previously-used table [4]. However, as the number of optical traps increases, each trap becomes weaker, leading to an increased lag in movement, and an in- creased likelihood of dropping the trapped object, if moved too rapidly. As the tablet’s touch interface is an absolute po- sition interface, where the finger obscures the object in the trap, it is important to consider designs which provide appro- priate feedback to the user to allow them to control the system efficiently. The issue of occlusion in touch interaction is well-known in mobile HCI, e.g. [11], but the challenges in this application differ from the typical occlusion problems in HCI. In the op- tical tweezer application, users are often dragging several ob- jects in traps to a particular location, in order to create a spe- cific geometric structure, where they are focussed on the goal,